Alkaline Chemistry in Crystallography

#### **Chapter 5**

## Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction in Annealed Crystals

*Yohichi Kohzuki*

#### **Abstract**

Combination method of strain-rate cycling tests and application of ultrasonic oscillations was conducted for KCl:Sr2+ (0.035, 0.050, 0.065 mol.% in the melt) single crystals at low temperatures. The measurement of strain-rate sensitivity (*λ*) of flow stress under the application of ultrasonic oscillatory stress provides useful information on the interaction between a mobile dislocation and impurities (Sr2+ ions) during plastic deformation and the variation of *λ* with stress decrement (Δ*τ*) due to oscillation has stair-like shape: The first plateau place ranges below the first bending point (*τ*p1) at low stress decrement and the second one extends from the second bending point (*τ*p2) at high stress decrement. The value of *λ* decreases with the Δ*τ* between the two bending points. The *τ*p1 is considered to represent the effective stress due to impurities when a dislocation begins to break-away from the impurities with the help of thermal activation during plastic deformation. Annealing the impure crystal by heat treatment, *τ*p1 decreases obviously at low temperature and the critical temperature *T*c, at which *τ*p1 is zero, also becomes slightly smaller. Furthermore, it was investigated whether a change in the state of a small amount of impurities has an influential factor of the flow parameters (e.g., the activation energy, the density of forest dislocations) from the data analyzed in terms of Δ*τ* vs. *λ* curve.

**Keywords:** heat treatment, dislocation, ultrasonic oscillatory stress, activation energy, divalent ion

#### **1. Introduction**

A large number of investigations on strength of materials have been made with alkali halide crystals so far [1–4]. This will be because the crystals have some advantages [5]: A small number of glide systems on account of rock salt structure, low dislocation density in grown crystal (e.g., 104 cm2 order for NaCl [6] and KCl [7] single crystals) as against that of annealed metals (around 107 cm2 [8], e.g., 1 to 5 106 cm2 in pure Cu single crystals [9], 1 to 5 106 cm2 in *α*-brass single crystals [10], and 107 cm2 order for pure Fe single crystals [11]), transparency, and further readily available single crystal, etc. Alkali halide crystals, therefore, are excellent materials for an investigation on mechanical properties of crystals.

#### *Alkaline Chemistry and Applications*

When alkali halide crystals are doped with divalent impurities (divalent cations), the impurities induce positive ion vacancies in order to conserve the electrical neutrality and are expected to be paired with the vacancies. They are often at the nearest neighbor sites forming a divalent impurity-vacancy (I-V) dipole, which attract them strongly [12], for crystals quenched from a high temperature. Then asymmetrical distortions are produced around the I-V dipoles.

The greatest hardness occurs for the highest concentration of isolated I-V dipole and the decrease of hardness is attributed to precipitation in KCl:Sr2+ single crystals [13]. The state of impurities in a crystal affects its hardness. Chin et al. have obtained the following experimental results: KCl:Sr2+ (840 ppm) crystals soften on annealing at temperatures up to 773 K; the hardness of NaCl:Ca2+ (below 2000 ppm) crystals starts to increase rapidly after aging at 100 °C for 30 min and decreases on further annealing [14]. And also, it was reported that the change in their state strongly influences the resistance to movement of the dislocations when a small amount of impurities aggregates or diffuses into the crystal in various impure alkali halide crystals (e.g., NaCl: Ca2+ or Mn2+, KCl: Sr2+ or Ba2+, LiF:Mg2+) by heat treatment [15]. Annealing an impure crystal by heat treatment, this will lead to the change in various deformation characteristics.

Mobile dislocations on a slip plane interact strongly only with these defects lying within one atom spacing of the glide plane [16]. Dislocation motions are related to the plasticity of crystal in a microscopic viewpoint. Solution hardening depends on the dislocation motion hindered by the atomic defects around impurities in crystals and is namely influenced by the dislocation-point defects interaction. Annealing KCl:Sr2+ single crystals here, the study on the interaction between a dislocation and impurities was made by the combination method of strain-rate cycling tests and application of ultrasonic oscillations. This method seems to provide the information on dislocation-impurities interaction in ionic crystal during plastic deformation [17, 18].

#### **2. Combination method of strain-rate cycling tests and application of ultrasonic oscillations**

KCl:Sr2+ (0.035, 0.050, 0.065 mol.% in the melt) specimens were prepared by cleaving the single crystalline ingots to the size of 5 5 15 mm3 . The specimens were kept immediately below the melting point for 24 h and were gradually cooled to room temperature at a rate of 40 Kh1 . This heat treatment was carried out for the purpose of reducing dislocation density as much as possible. Further, the specimens were held at 673 K for 30 min and were rapidly cooled by waterquenching in order to disperse the impurities (Sr2+) in the specimens immediately before the following tests. The specimen KCl:Sr2+ (0.050 mol.%) is termed the quenched specimen in this article.

The specimens were compressed along the <100> axis at 77 K to the room temperature and the ultrasonic oscillatory stress (*τ*v) was intermittently superimposed in the same direction as the compression. The strain-rate cycling test under the ultrasonic oscillation is illustrated in **Figure 1**. Superposition of oscillatory stress causes a stress drop (Δ*τ*) during plastic deformation. The strain-rate cycling between strain-rates of *ε*\_ 1 (2.2 105 s 1 ) and *ε*\_2 (1.1 104 s 1 ) was performed keeping the stress amplitude of *τ*v constant. Then, the variation of stress due to the strain-rate cycling is Δ*τ*'. The strain-rate sensitivity (Δ*τ*'/Δln*ε*\_) of the flow stress, which is given by Δ*τ*'/1.609, was used as a measurement of the strain-rate sensitivity (*λ*). The details was described in the article [19].

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

**Figure 1.**

*Explanatory diagram of a change in applied shear stress, τ*a *, for the strain-rate cycling test between the strain rates, ε*\_1 *(2.2 105 s 1 ) and ε*\_2 *(1.1 104 s 1 ), under superposition of ultrasonic oscillatory shear stress, τ*v*.*

#### **3. Effective stress due to agglomerates in the crystals**

Relation between Δ*τ* and *λ* for KCl:Sr2+ (0.050 mol.% in the melt) at the shear strain of 10% is shown in **Figure 2**. The measuring temperature is 103 K. Δ*τ* vs. *λ* curve reflects the effect of ultrasonic oscillation on the dislocation motion on the slip plane containing many weak obstacles such as impurities and a few forest dislocations during plastic deformation [20]. Open circles in the figure represent the relation for the specimen quenched from 673 K to room temperature, and open triangles for the specimen obtained by storing the quenched specimen at room temperature for a half year. The second one is termed the stored specimen here. As can be seen from **Figure 2**, the variation of *λ* with Δ*τ* has stair-like shape (there are two bending points on each curve, and there are two plateau regions: the first plateau region ranges below the first bending point (*τ*p1) at low stress decrement and the second one extends from the second bending point (*τ*p2) at high stress decrement) for the quenched specimen. *λ*

**Figure 2.**

*Relation between the strain-rate sensitivity (λ) and the stress decrement (Δτ) at 103 K for (○) the quenched specimen and (*Δ*) the stored specimen.*

**Figure 3.** *Dependence of (○) τp1 and (*Δ*) τp2 on temperature for (*a*) the quenched specimen and (b) the stored specimen.*

decreases with the stress decrement between the two bending points. *λ*p denoted in **Figure 2** is introduced in section 6 of this article. The *τ*p1 (Δ*τ* value at the first bending point) for the stored specimen is smaller than that for the quenched specimen. *τ*p1 is considered to be the effective stress due to the impurities (Sr2+) which lie on the dislocation when a dislocation begins to break-away from the impurities with the help of thermal activation during plastic deformation, because the value of *τ*p1 has been reported to be depend on temperature (*τ*p1 shifts in the direction of lower Δ*τ* as the temperature becomes larger), and on the type and the density of weak obstacle [21]. The *τ*p2 (Δ*τ* value at the second bending point) for the stored specimen, however, does not appear within the measurement, because high stress amplitude could not be applied to the specimen during the strain-rate cycling tests. Furthermore, *τ*p1 and *τ*p2 of the quenched and the stored specimens were investigated at various temperatures. **Figures 3(a)** and **(b)** show the results for the quenched specimen and for the stored one, respectively. The *τ*p1 for the stored specimen is smaller than that for the quenched specimen at a given temperature. On the other hand, the *τ*p2 for the stored specimen is a little smaller than that for the quenched one as compared with *τ*p1. In the following sections of this article, the state of impurities in the specimen is clarified and its influence on the interaction between a dislocation and the impurities will be described.

### **4. State of impurities (Sr2+) in KCl:Sr2+ single crystals**

Measurements of the I-V dipole concentration and of the flow stress for KCl:Sr2+ (0.023 mol.%) were reported as a function of annealing time at 403 K after

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

quenching from 673 K by Dryden et al. [15]. Observing the concentration of isolated I-V dipoles in the crystal before and after the annealing and also the change of flow stress with it, the dipole concentration decreases and the flow stress becomes lower at a longer annealing time above 10 h.

Here is clarified the state of impurities in the specimen and is referred to the influence of the state of impurities on the dislocation-impurities interaction, especially on the relation between temperature and the effective stress. The crystals used here are two kinds of specimens. The first is KCl:Sr2+ (0.050 mol.% in the melt) at the preceding section 3 and is named the quenched specimen. The second is prepared by keeping the quenched specimen at 370 K for 500 h and gradually cooling in a furnace for the purpose of aggregating impurities in it [22]. This is hereafter termed the annealed specimen.

Dielectric absorption of an I-V dipole causes a peak on the tan*δ*-frequency relation. A relative formula which gives the proportionality between the concentration of I-V dipoles and a Debye peak height is expressed by [23].

$$\tan \delta = \frac{2\pi e^2 c}{3\epsilon' akT}, (\text{maximum}) \tag{1}$$

where *e* is the elementary electric charge, *c* the concentration of the I-V dipole, *ε*0 the dielectric constant in the matrix, *a* the lattice constant and *kT* has its usual meaning.

**Figure 4** shows the influence of this heat treatment on the tan*δ*-frequency curves for KCl:Sr2+ at 393 K. The upper solid and dotted curves correspond to the

**Figure 4.**

*Dielectric loss in KCl:Sr2+ (0.050 mol.% in the melt) at 393 K: (○) for the quenched specimen, (*Δ*) for the annealed specimen. (*� � *- -) losses coming from the dipoles (reproduced from ref. [24] with permission from the publisher).*

quenched specimen and the lower curves the annealed specimen. Dotted lines show Debye peaks obtained by subtracting the d.c. part which is obtained by extrapolating the linear part of the solid curves in the low-frequency region to the highfrequency region. Introducing the peak heights of the dotted curves into Eq. (1), the concentration of the isolated dipole was determined to be 98.3 ppm for the quenched specimen and 21.8 ppm for the annealed specimen. On the other hand, the atomic absorption gave 121.7 ppm for the Sr2+ concentration in the quenched specimen and 96.2 ppm for the annealed specimen. Therefore, it should be considered that 71.9% of the I-V dipoles turn into the aggregates in KCl:Sr2+ single crystal and form at least trimers [22] by the heat treatment. The trimers (Sr2+-vacancy-Sr2+ vacancy-Sr2+-vacancy) have a structure in which three dipoles are arranged hexagonally head to tail in a (111) plane, as suggested by Cook and Dryden [22].

#### **5. Effective stress and critical temperature**

**Figure 5** shows the variation of *λ* with Δ*τ* at a shear strain of 8% for the annealed specimen at 125 K. The curve in the **Figure 5** has two bending points and two plateau regions at a given shear strain and temperature. The first plateau region ranges below the first bending point at low stress decrement and the second one extends from the second bending point at high stress decrement. The *λ* decreases with the Δ*τ* between the two bending points. The values of Δ*τ* at the first and the second bending points are referred to as *τ*p1 and *τ*p2, respectively.

The dependence of *τ*p1 and *τ*p2 on temperature for the quenched specimen is shown in **Figure 6(a)**, while that for the annealed specimen is shown in **Figure 6(b)**. The value of *τ*p1 becomes small by forming into the aggregates in the crystal and this result is clearer at lower temperature. This may be caused by the result that the separation between the weak obstacles lying on the mobile dislocation becomes wider as the I-V dipoles turn into aggregates. In addition, it is supposed that the decrease in *τ*p1 due to agglomerate of the I-V dipoles, i.e. softening, would be attributable to the loss of tetragonality in terms of the Fleischer's model [16], as suggested by Chin et al.

#### **Figure 5.**

*Relation between the strain-rate sensitivity (λ) and the stress decrement (Δτ) at a shear strain of 8% for the annealed specimen at 125 K (reproduced from ref. [24] with permission from the publisher).*

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

**Figure 6.**

*Dependence of (○) τp1 and (*Δ*) τp2 on temperature for KCl:Sr2+ (0.050 Mol.% in the melt): (a) for the quenched specimen and (b) for the annealed specimen (reproduced from ref. [24] with permission from the publisher).*

[14]. The decrease in the effective stress due to agglomerates of I-V dipoles has been reported for alkali halide crystals doped with divalent cations so far [14, 15, 25–27].

As for *τ*p2, no great difference is seen for the two kinds of specimens. Accordingly, as the I-V dipoles turn into the aggregates by the heat treatment, the difference between *τ*p1 and *τ*p2 obviously becomes wider at lower temperature. This may be caused by the wider distribution of Sr2+ obstacles on mobile dislocation in the annealed specimen as against that in the quenched one.

The critical temperature *T*c at which *τ*p1 is zero and a dislocation breaks away from the impurities only with the help of thermal activation is around 210 K for the annealed specimen. This *T*c value is small in contrast to *T*c≈240 K of the quenched specimen as can be seen from **Figures 6(a)** and **(b)**.

#### **6. Activation energy for the dislocation breaking-away from the defects**

The thermally activated deformation rate (*ε*\_) is expressed by an Arrhenius-type equation:

$$
\dot{\varepsilon} = \dot{\varepsilon}\_0 \exp\left(\frac{-\Delta G}{kT}\right),
\tag{2}
$$

where *ε*\_0 is a frequency factor and is unique for a particular dislocation mechanism. And the change in Gibbs free energy, Δ*G*, is expressed for square forcedistance relation (SQ) between a dislocation and an impurity as follows.

$$
\Delta G = \Delta G\_0 - \tau^\* Lbd,\tag{3}
$$

where Δ*G*0 is the Gibbs free energy for the break-away of the dislocation from the impurity in the absence of an applied stress, *τ* \* the effective shear stress due to the impurities, *L* the length of dislocation, *b* the magnitude of the Burgers vector and *d* the activation distance. The Gibbs free energy for the SQ is given by

$$
\Delta G = \Delta G\_0 - \beta \tau^{\*2/3} \tag{4}
$$

$$
\beta = \left(2\mu b^4 d^3 L\_0^2\right)^{1/3},\tag{5}
$$

where *μ* is the shear modulus and *L*0 the average spacing of impurities on the slip plane. Differentiating the substitutional equation of Eqs. (4) and (5) in Eq. (2) with respect to the shear stress, we find

$$
\pi\_{p1} \left( \frac{\partial \ln \dot{\varepsilon}}{\partial \tau^\*} \right) = \left( \frac{2 \Delta G\_0}{3kT} \right) + \frac{2}{3} \ln \left( \frac{\dot{\varepsilon}}{\dot{\varepsilon}\_0} \right). \tag{6}
$$

The Gibbs free energy for the interaction between a dislocation and the impurity is expressed from the Eq. (2), namely

$$
\Delta G = akT, \left( a = \ln \left( \frac{\dot{\varepsilon}\_0}{\dot{\varepsilon}} \right) \right) \tag{7}
$$

and the Δ*G* for the Fleischer's model [16] taking account of the Friedel relation [28] (F-F) is also expressed by

$$
\Delta G = \Delta G\_0 \left\{ 1 - \left( \frac{\tau^\*}{\tau\_0^\*} \right)^{1/3} \right\}^2, (\Delta G\_0 = F\_0 b) \tag{8}
$$

where *τ* \* 0 is the value of *τ* \* at absolute zero and *F*0 the force acted on the dislocation at 0 K. It is well known that the Friedel relation [28] between the effective stress and the average length of dislocation segments can be applied to most weak obstacles to dislocation motion at low solute concentration. Differentiating the combination of Eqs. (7) and (8) with respect to the shear stress gives

$$\frac{\partial \ln \dot{\varepsilon}}{\partial \tau^\*} = \left(\frac{2\Delta G\_0}{3kT\tau\_0^\*}\right) \left(\frac{\tau\_0^\*}{\tau^\*}\right)^{2/3} \left\{ 1 - \left(\frac{\tau^\*}{\tau\_0^\*}\right)^{1/3} \right\} + \frac{\partial \ln \dot{\varepsilon}\_0}{\partial \tau^\*}.\tag{9}$$

Further, substituting the following Eq. (10) in Eq. (9) gives Eq. (11)

$$\left(\frac{\tau\_{p1}}{\tau\_{p0}}\right)^{1/3} = \mathbf{1} - \left(\frac{T}{T\_c}\right)^{1/2} \tag{10}$$

namely,

$$\frac{\partial \ln \dot{\varepsilon}}{\partial \tau^\*} = \left(\frac{2\Delta G\_0}{3kT\tau\_{p0}}\right) \left\{ 1 - \left(\frac{T}{T\_c}\right)^{1/2} \right\}^{-2} \left(\frac{T}{T\_c}\right)^{1/2} + \frac{\partial \ln \dot{\varepsilon}\_0}{\partial \tau^\*},\tag{11}$$

where *τ* \* 0 is replaced by *τ*p0 (*τ*p1 value at absolute zero). Eq. (10) represents the relative formula of *τ*p1 and temperature, which will reveal the force-distance

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

**Figure 7.**

*Linear plots of Eq. (11) for the quenched specimen: KCl:Sr2+ ((○) 0.035 mol.%, (*Δ*) 0.050 mol.%, (*□*) 0.065 mol.% in the melt) (reproduced from ref. [30] with permission from the publisher).*

**Figure 8.**

*Linear plots of Eq. (6) for the annealed specimen (reproduced from ref. [31] with permission from the publisher).*

relation between a dislocation and an impurity. The result of calculations of Eq. (11) for the F-F is shown in **Figure 7** for the quenched specimen. The open symbols correspond to the inverse of *λ*p for the quenched specimen, which are derived from the difference between *λ* values at first plateau place and at second one on the relative curves of Δ*τ* vs. *λ* as shown in **Figure 2**. *λ*p is considered to be due to the impurities [29]. On the basis of the slope of straight line, the Gibbs free energy is 0.39 eV. The F-F is suitable for the force-distance profile for the quenched specimen but not for the annealed one [30, 31]. As for the annealed specimen, SQ seems to be most suitable of the three profiles: a square force-distance relation, a parabolic one and a triangular one, taking account of the Friedel relation [31]. The Gibbs free energy for the interaction between a dislocation and the aggregate in the annealed specimen, which is obtained through the slope of straight line in **Figure 8**, is 0.26 eV. The open circles in **Figure 8** show the results of calculations for Eq. (6). Here, the *∂* ln *ε*\_ *∂τ* ∗ in Eq. (6) is obtained from *λ*p. The Gibbs free energy for the annealed specimen is smaller than that of the quenched one.

#### **7.** *λ* **at second plateau place on Δ***τ* **vs.** *λ* **curve**

Influence of the heat treatment on the density of forest dislocations is treated in detail for the two kinds of KCl:Sr2+ (0.050 mol.% in the melt) single crystals (i.e., the quenched and the annealed specimens). This is examined from the variation of *λ* (see the marked part with gray circle in **Figure 9**) at the second plateau place on the Δ*τ* vs. *λ* curve with shear strain, where the obstacles to the dislocation motion are only forest dislocations and the impurities no longer act as obstacles [21]. A general Δ*τ* vs. *λ* at a given strain is schematically drawn in **Figure 9**, where the curve has two bending points and two plateau regions. The *λ* at second plateau place on the Δ*τ* vs. *λ* curve is considered to be due to dislocation cuttings [21]. The variation of *λ* at second plateau place with shear strain (Δ*λ*/Δ*ε*) is considered to be the increase of forest dislocation density with the shear strain [32]. In **Figure 10**, the Δ*λ*/Δ*ε* dependence of temperature is shown in the different plastic deformation regions of stress–strain curve for the two kinds of specimens: the quenched and the annealed

#### **Figure 9.**

*Illustration of relationship between the strain-rate sensitivity (λ) of flow stress and the stress decrement (Δτ) at a given strain, ε. (ε*1 <*ε*2*).*

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

**Figure 10.**

*Dependence of Δ λ/Δε on the temperature in the different plastic deformation regions: (○) for the quenched specimen and (*●*) for the annealed specimen in stage I; (*Δ*) for the quenched specimen and (*▲*) for the annealed specimen in stage II (reproduced from ref. [32] with permission from the publisher).*

specimens. Δ*λ*/Δ*ε* in stage I (easy glide region) and in stage II (linear hardening region) are represented by a circle and a triangle, respectively. The open symbols correspond to that for the quenched specimen and the solid ones that for the annealed specimen. The curves in **Figure 10** are to guide the reader's eye. Unfortunately, the Δ*λ*/Δ*ε* could not be obtained at low temperature. Three-stage strain hardening is obtained for KCl [33, 34] and impure KCl single crystals doped with monovalent or divalent cations [35, 36]. Two phenomena are observed for both the specimens in **Figure 10**. That is, the first phenomenon is that the Δ*λ*/Δ*ε* in stage II is obviously larger than that in stage I at a given temperature. The other phenomenon is that the Δ*λ*/Δ*ε* in stage I and in stage II increases with decreasing temperature. **Figure 10** also shows that the Δ*λ*/Δ*ε* for the annealed specimens is considerably large in contrast to that for the quenched specimens in the two stages within the temperature. This may result from a rapid increase in forest dislocation density with shear strain in the annealed specimen. Accordingly, the increase in forest dislocation density in the annealed specimen seems to be remarkable in the two stages under the compression test, compared with it in the quenched specimen.

#### **8. Conclusions**

The following conclusions were derived from the data analyzed in terms of the Δ*τ* vs. *λ* curves for KCl:Sr2+ single crystals.

1.The plots of Δ*τ* vs. *λ* have two bending points and two plateau regions for the quenched specimen and similar result is observed also for the specimen stored at room temperature for a half year. On the basis of the relative curve of Δ*τ* vs. *λ*, it was found that *τ*p1 due to the impurities in the stored specimen is smaller than that of the quenched specimen within the temperatures.

### **Conflict of interest**

The author declares no conflict of interest.

### **Author details**

Yohichi Kohzuki

Department of Mechanical Engineering, Saitama Institute of Technology, Fukaya, Japan

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

#### **References**

[1] Urusovskaya AA, Darinskaya EV, Voszka R, Jansky J. Defect structure and the nature of the obstacles for dislocations in NaCl(Ca) crystals. Kristall und Technik. 1981;**16**:597-601. DOI: https://doi.org/10.1002/ crat.19810160511

[2] Suszyńska M, Nowak-Woźny D. Mechanical characteristics of the NaCl: Eu2+ crystal system. Crystal Research and Technology. 1990;25: 855–861. DOI: https://doi.org/10.1002/ crat.2170250721

[3] Boyarskaya YS, Zhitaru RP, Palistrant NA. The anomalous behaviour of the doped NaCl crystals compressed at low temperatures. Crystal Research and Technology. 1990; 25:1469–1473. DOI: https://doi.org/ 10.1002/crat.2170251219

[4] Sprackling MT. The plastic deformation of simple ionic crystals. In: Alper AM, Margrave JL, Nowick AS, editors. Materials Science and Technology. London New York San Francisco: Academic Press; 1976.

[5] Kataoka T. Studies on plastic deformation of alkali halide crystals [thesis]. Osaka: Osaka University; 1975, p 2 (in Japanese).

[6] Argon AS, Nigam AK, Padawer GE. Plastic deformation and strain hardening in pure Nacl at low temperatures. Philosophical Magazine. 1972;25:1095–1118. DOI: https://doi.org/ 10.1080/14786437208226855

[7] Kohzuki Y. Studies on interaction between a dislocation and impurities in KCl single crystals [thesis]. Kanazawa: Kanazawa University; 1994, pp 12–13.

[8] Suzuki H. Introduction to Theory of Dislocations. Tokyo: AGNE; 1989, p 70 (in Japanese).

[9] Young Jr. FW. Etch pits at dislocations in copper. Journal of Applied Physics. 1961;32:192–201. DOI: https://doi.org/10.1063/1.1735977

[10] Meakin JD, Wilsdorf HGF. Dislocations in deformed single crystals of alpha brass. I. General observations. Transactions of the American Institute of Mining, Metallurgical and Petroleum Engineers. 1960;218:737–745.

[11] Takeuchi S. Solid-Solution Strengthening in Single Crystals of Iron Alloys. Journal of the Physical Society of Japan. 1969;27:929–940. DOI: https:// doi.org/10.1143/JPSJ.27.929

[12] Pick H, Weber H. Dichteänderung von KCl-Kristallen durch Einbau zweiwertiger Ionen. Zeitschrift für Physik. 1950;128:409–413. DOI: https:// doi.org/10.1007/BF01339441

[13] Green M. L, Zydzik G. Effect of heat treatment on the microhardness of some mixed and doped alkali halides. Scripta Metall. 1972;6:991–994. DOI: https://doi. org/10.1016/0036-9748(72)90159-7

[14] Chin G. Y, Van Uitert L. G, Green M. L, Zydzik G. J, Kometani T. Y. Strengthening of alkali halides by divalent-ion additions. J. Am. Ceram. Soc. 1973;56:369–372. DOI: https://doi.org/10.1111/j.1151-2916.1973. tb12688.x

[15] Dryden J. S, Morimoto S, Cook J. S. The hardness of alkali halide crystals containing divalent ion impurities. Philosophical Magazine. 1965;12:379– 391. DOI: https://doi.org/10.1080/<PHONE_NUMBER>8218880

[16] Fleischer RL. Rapid solution hardening, dislocation mobility, and the flow stress of crystals. Journal of Applied Physics. 1962;33: 3504–3508. DOI: https://doi.org/ 10.1063/1.1702437

[17] Ohgaku T, Takeuchi N. The relation of the Blaha effect with internal friction for alkali halide crystals. Physica Status Solidi A. 1988;105:153–159. DOI: https:// doi.org/10.1002/pssa.2211050115

[18] Ohgaku T, Takeuchi N. Relation between plastic deformation and the Blaha effect for alkali halide crystals. Physica Status Solidi A. 1989;111:165– 172. DOI: https://doi.org/10.1002/ pssa.2211110117

[19] Kohzuki Y. Study on dislocationdopant ions interaction during plastic deformation by combination method of strain-sate cycling tests and application of ultrasonic oscillations. In: Singh D, Condurache-Bota S, editors. Electron Crystallography. IntechOpen: London; 2020. DOI: 10.5772/intechopen.92607

[20] Kohzuki Y. Bending angle of dislocation pinned by an obstacle and the Friedel relation. Philosophical Magazine. 2010;90:2273–2287. DOI: 10.1080/14786431003636089

[21] Kohzuki Y, Ohgaku T, Takeuchi N. Interaction between a dislocation and impurities in KCl single crystals. Journal of Materials Science. 1993;28:3612–3616. DOI: https://doi.org/10.1007/ BF01159844

[22] Cook J. S, Dryden J. S. An investigation of the aggregation of divalent cationic impurities in alkali halides by dielectric absorption. Proceedings of the Physical Society. 1962;80:479–488. DOI: 10.1088/ 0370-1328/80/2/315

[23] Lidiard A. B. Handbuch der Physik, Berlin: Springer; 1957, Vol. 20, p 246.

[24] Kohzuki Y, Ohgaku T, Takeuchi N. Influence of a state of impurities on the interaction between a dislocation and impurities in KCl single crystals. Journal of Materials Science. 1993;28:6329–6332. DOI: https://doi.org/10.1007/ BF01352192

[25] Johnston, W. G. Effect of Impurities on the Flow Stress of LiF Crystals. Journal of Applied Physics. 1962;33; 2050–2058. DOI: https://doi.org/ 10.1063/1.1728892

[26] Gaiduchenya V. F, Blistanov A. A, Shaskolskaya M. P. Thermally activated slip in LiF crystals. Soviet Physics Solid State 1970;12;27–31.

[27] Buravleva M. G, Rozenberg G. K, Soifer L. M, Chaikovskii E. F. Changes in the flow stress of LiF:Mg2+ and LiF: Co2+ crystals during precipitation of solid solutions. Soviet Physics Solid State. 1980;22;150–152.

[28] Friedel J. Dislocations, Oxford: Pergamon Press; 1964, p 224.

[29] Kohzuki Y, Ohgaku T, Takeuchi N. Interaction between a dislocation and various divalent impurities in KCl single crystals. Journal of Materials Science. 1995;30:101–104. DOI: https://doi.org/ 10.1007/BF00352137

[30] Kohzuki Y. Study on the interaction between a dislocation and impurities in in KCl:Sr2+ single crystals by the Blaha effect Part IInteraction between a dislocation and impurity for the Fleischer's model taking account of the Friedel relation. Journal of Materials Science. 2000;35:3397–3401. DOI: https://doi.org/10.1023/A:<PHONE_NUMBER>796

[31] Kohzuki Y, Ohgaku T. Study on the interaction between a dislocation and impurities in in KCl:Sr2+ single crystals by the Blaha effect Part IIInteraction between a dislocation and aggregates for various force-distance relations between a dislocation and an impurity. Journal of Materials Science. 2001;36:923–928. DOI: https://doi.org/10.1023/A:<PHONE_NUMBER>566

[32] Kohzuki Y. Study on the interaction between a dislocation and impurities in KCl:Sr2+ single crystals by the Blaha

*Study on Influence of a State of Dopants on Dislocation-Dopant Ions Interaction… DOI: http://dx.doi.org/10.5772/intechopen.96395*

effect-Part IV influence of heat treatment on dislocation density. Journal of Materials Science. 2009;44: 379–384. DOI: https://doi.org/10.1007/ s10853-008-3150-8

[33] Alden T. H. Latent hardening and the role of oblique slip in the strain hardening of rock-salt structure crystals. Transactions of the Metallurgical Society of AIME. 1964;230:649–656.

[34] Davis L. A, Gordon R. B. Plastic deformation of alkali halide crystals at high pressure: Work-hardening effects. Journal of Applied Physics. 1969;40: 4507–4513. DOI: https://doi.org/ 10.1063/1.1657224

[35] Kohzuki Y. Interaction between a dislocation and impurities in KCl doped with Li+ or Na+ . Journal of Materials Science. 2000;35:2273–2277. DOI: https://doi.org/10.1023/A:<PHONE_NUMBER>091

[36] Kohzuki Y. Influence of various divalent impurities on dislocation density in KCl:Mg2+, Ca2+, Sr2+ or Ba2+ single crystals. Journal of Materials Science. 2003;38:953–958. DOI: https:// doi.org/10.1023/A:1022373124795

#### **Chapter 6**

## Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide Materials

*Amira Marzouki, Ameni Brahmia, Riadh Marzouki, Mosbah Jemmali, Ismat H. Ali and Mohamed Faouzi Zid*

#### **Abstract**

In this chapter, the correlation between structure and electrical properties of Na2*M*P1.5As0.5O7 (*MII* = Co and Cu) are treated. The structural study shows that the cobalt and copper isotype materials can be crystallized in the tetragonal and monoclinic systems, respectively. The electrical study using impedance spectroscopy technique showed that these mixed diphosphate diarsenates are fast electrical conductors; however, the cobalt material exhibited more conductive property than the copper compound. In addition, the powder perovskite manganites La0.7*M*0.2*M*'0.1MnO3 (*M* = Sr, Ba and *M*' = Na, Ag and K) have been prepared using the conventional solid-state reaction. The structural, magnetic, and magnetocaloric properties of these perovskite manganites compounds were studied extensively by means of X-ray powder diffraction (XRD) and magnetic measurements. These samples were crystallized in the distorted rhombohedral system with R3c space group. The variation of magnetization (M) vs. temperature (T) reveals that all compounds exhibit a second-order ferromagnetic to paramagnetic phase transition in the vicinity of the Curie temperature (TC). A maximum magnetic entropy change, Δ*SM Max*, of 4.07 J kg1 K1 around 345 K was obtained in La0.7Sr0.2Na0.1MnO3 sample upon a magnetic field change of 5 T. The Δ*SM Max* values of La0.7Ba0.2M'0.1MnO3 are smaller in magnitude compared to La0.7Sr0.2*M'*0.1MnO3 samples and occur at lower temperatures.

**Keywords:** diphosphate-diarsenate, crystal structure, electrical properties, perovskite, magnetic materials, magnetocaloric effect

#### **1. Introduction**

The exploration of new alkali-based materials, especially Na-ion compounds, has become an area of intense activity [1–3]. In fact, these materials have the potential to replace lithium-based cathodes in the new generation of batteries. This trend can be explained by the global increase in demand for lithium and its toxicity compared to the low cost of sodium and its abundance in nature [4].

The two main methods of developing new cathodes, which are currently being explored, are either by researching new crystalline materials or by improving known materials by improving their electrical properties and electrochemical performance. In either case, crystallography remains the key to the development of these electrochemical systems as a determination of crystal structure and ion transport followed by electrochemical properties.

In this context, the exploration and investigation of phosphates, arsenates, and molybdates of transition metals and monovalent cations (Li, Na, K, Ag, etc.) have a promising field for various applications: electrical, piezoelectric processes, ferroelectric, magnetic, catalytic [5, 6]. Moreover, taking into account their remarkable structural richness, in particular the melilite structure [7], the olivine structure [8], and the sodium super ionic conductor (NaSICON) structure [9], these materials show several interesting physical properties, in particular, the ionic conduction and ion exchange [8, 9]. Many sodium-based materials have recently been prepared and tested for their electrical and/or electrochemical properties, including Na2CoP2O7 [10], LiCoAsO4 [11], Na1.86Fe3(PO4)3 [12], etc. These physicochemical properties are linked on the one hand to their structural wealth and on the other hand to the degree of openness of their anionic frameworks which can be dense, open, or microporous.

Thus, the investigation of this type of material requires a good correlation between crystal structure and electrical properties taking into account factors influencing the electrical conductivity such as porosity and the temperature range of stability of the crystal structure of the sample. In this chapter, the first part comprises a structural study of alkali metals (especially Na elements) transition metals (Co, Cu) phosphates-arsenates and their correlations with electrical properties.

On the other hand, alkaline atoms have an electronic procession, composed of a set of inert internal layers, having the structure of a rare gas, and an additional electron, or valence electron, which orbiting an s-type orbital. The study of ionization potentials shows that this electron is easily torn off, the heavier the atom is the lesser the energy is required for ionization. The chemistry of alkali metals is essentially constituted by the study of the transition to the ionized state M+ and by the properties of this ion. Most alkaline compounds, therefore, have a purely ionic structure; this peculiarity, together with the fact that the most stable ionic edifices are constituted by ions of similar volume, makes it possible to predict that as a general rule hydrides, nitrides, carbides, and simple oxides, that is to say compounds possessing fairly small anions, will be all the more stable the lighter the alkali metal, while the salts corresponding to large anions, peroxides (OdO)2, superoxides (OdO), oxacids, and halides will be more stable with heavy alkali metals.

The dangers of the impact of synthetic refrigerants on the environment are central to the global ecological scene. Global warming, by its complexity and magnitude, poses several challenges to the ecological future of the earth. One of the solutions proposed to slow down this process is to reduce the production of greenhouse gases. While demand is growing, the areas of refrigeration and air conditioning are trying to renew themselves to meet the new ecological requirements. Such developments will lead to new technologies applicable to domestic and industrial uses of microtechnologies, HVAC systems refrigerators, heat pumps and affecting the automotive, railway, aeronautical, and aerospace industries. Magnetocaloric cooling (MCE) is a possible solution and becomes a promising attempt. Magnetic refrigeration is a technology that relies on the magnetocaloric effect, similar to compressibility refrigerant for gas refrigeration.

The magnetocaloric effect being present in all magnetic substances gives a large field of research activity to find active materials suitable for every utility [13, 14]. Gadolinium is the reference material for magnetic cold at room temperature. This

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

element also has the advantage of being an easy compound to obtain through its purity. It finds its technological development, thanks to the ease of implementation reflected by its high ductility and suitability. This is why it is used in most current magnetic refrigeration devices at room temperature. However, its prohibitive price (up to 3500 €/kg) and its limited reserves preclude it from possible magnetocooling materials for consumer applications. It is, therefore, important to find another magnetocaloric material. Manganese is an energetically clean alternative to address this problem and may be potential material for magnetic refrigeration.

The compounds LnMnO3, Ln2MnO4 (where Ln is a rare earth) are antiferromagnetic insulators. The partial substitution of Ln3+ by a divalent element A2+ (A2+ = Ca2+; Sr2+; Ba2+; Pb2+ … ), monovalent A+ (A+ = Ag+ , Na+ , K+ … ), trivalent A3+ (A3+ = Nd3+, Sm3+, Pr3+ … ), or a gap in Ln1�xAxMnO3 results in oxidation or partial reduction of Mn3+/Mn4+ ions, a structural transition (cubic \$ rhombohedral \$ orthorhombic) and an antiferromagnetic order change TN = 125 K in LaMnO3) ferromagnetic with Tc > 360 K depending on the composition [15–22].

The manganese oxides, which we are interested in given the importance of their electrical and magnetic properties [23, 24], crystallize in a perovskite-like structure. For this structure, of general formula ABO3, the Bravais network of sites B (octahedral site), whose species are generally transition cations (Mn3+, Mn4+, Ti4+, Al3+ … ) occupying the eight vertices of the cube, is simple cubic. Oxygen ions occupy the midpoints of the ridges, and species A in coordination 12 (cuboctahedral site) occupying the center of the cube are alkaline ions (K+ , Na+ , Ag+ … ). Alkaline earth (Sr2+, Ca2+, Pb2+ … ) or rare earths (La3+, Pr3+, Nd3+ … ). Site B is, therefore, occupied by an ion of octahedral coordination, manganese in the case of manganites, thus forming the MnO6 octahedra (**Figure 1**(**a**) and (**b**)).

It should be noted that the cations A and B must allow the electroneutrality of the compound; that is, the sum of the charges of the cations A and B must be equal to the total charge of the anions. Three types of ternary oxides can be found: cation A is monovalent (A+ B5+ O2� 3), bivalent (A2+ B4+ O2� 3), or trivalent (A3+ B3+ O2� 3).

**Figure 1.** *Unit cell ABO3 (a), octahedral environment of species B and (b) cuboctahedral environment of species A.*

#### **2. Structure, characterization, and electrical properties of Na2***M***P1.5As0.5O7 (***MII* **= Co and Cu).**

#### **2.1 Structural characterizations**

The X-ray powder diffractograms were recorded in the range 10–70° at 20°C with 0.02° step (**Figures 2** and **3**). GSAS software [25] using Rietveld method was used to confirm the crystallinity and purity of the obtained powders. The final reliability factors are Rp = 1.4%, Rwp = 1.9%, and Rp = 5.4%, Rwp = 6.9%, of the Co and Cu samples, respectively.

In this case, the difference between the two diffractograms is noticeable. Indeed, the Na2CoP1.5As0.5O7 [26] material crystallizes in the tetragonal system of the space group P42/mnm with the unit cell parameters a = 7.764 (3) Å; c = 10.385 (3) Å.

**Figure 2.** *Rietveld refinement pattern of Na2CoP1.5As0.5O7 powder.*

**Figure 3.** *Rietveld refinement pattern of Na2CuP1.5As0.5O7 powder.*

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

While the compound Na2CuP1.5As0.5O7 [27] crystallizes in the monoclinic system of the C2/c space group with the unit cell parameters a = 14.798 (2) Å; b = 5.729 (3) Å; c = 8.075 (2) Å; β = 115.00 (3)°.

In fact, the structural unit of the compound Na2CoP1.5As0.5O7 is formed by a site occupied by a cobalt atom, a site occupied by a phosphorus atom partially substituted by arsenic, two sites for the sodium atoms and three sites for the sodium atoms ten oxygen atoms. The asymmetric unit is shown in **Figure 4**.

(i) y + 1/2, x 1/2, z + 1/2; (ii) x + 1, y, z; (iii) y + 1/2, x + 1/2, z + 1/2].

The projection of the structure in the b direction (**Figure 5**) illustrates the layered nature of the anionic framework with two alternating orientations of anion

**Figure 4.** *Structural unit of Na2CoP1.5As0.5O7 with atom labeling scheme.*

**Figure 5.** *Projection of the Na2CoP1.5As0.5O7 along* b *direction.*

**Figure 6.** *Structural unit projection of Na2CuP1.5As0.5O7.*

sheets [Co(P/As)2O7] 2 per unit cell parallel to the *ab* plane where sodium cations are sandwiched between layers. Compared to Na2CoP2O7 [28] crystal structure, the substitution of the phosphorus ions by arsenic cations decreases the size of the interlayer space and decreases in binding NadO distance.

On the other hand, the structural unit of Na2CuP1.5As0.5O7 (**Figure 6**) contains two P2O7 units connected by the corner with two CuO4 with square plane geometry. The charge neutrality of the structural unit is ensured by four sodium ions.

The Cu2P4O15 groups of the structural unit are linked by oxygen peaks to give infinite chains, wavy sawtooth in the [001] direction (**Figure 7**). Na+ ions are located in the inter-chain space.

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

#### **2.2 Electrical study of Na2CoP1.5As0.5O7**

The electrical properties of the cobalt compound were studied using complex impedance spectroscopy [29] in the temperature range from 240 to 360°C after stabilization at each step of 30°C between 1 Hz and 13 MHz frequency range. The electrical parameters are concluded using the conventional electrical circuit: R//CPE-R//CPE, where Rest is a resistance and CPE is a constant phase element:

The electrical parameters are summarized in **Table 1**.

The electrical measurements show that the electrical conductivity of Na2CoP1.5As0.5O7 increases from 4.0 106 S cm1 at 240°C to 3.69 105 S cm1 at 360°C. On the other hand, the activation energy which follows Arrhenius' law is 0.56 eV.

#### **2.3 Electrical study of Na2CuP1.5As0.5O7**

The Na2CuP1.5As0.5O7 sample was sintered at 550°C for 2 h with 5°C/min of heating and cooling. The relative density of the obtained pellet is D = 88%. Electrical measurements in the temperature range of 260–380°C were performed using complex impedance spectroscopy. The recorded spectra are shown in **Figure 8**.

The best refinements of impedance spectra were obtained when using a conventional electrical circuit Rg//CPEg-Rgb//CPEgb.

#### **Table 1.**

*Results of electrical parameters of Na2CoP1.5As0.5O7.*

**Figure 8.** *Impedance spectra of Na2CuP1.5As0.5O7 recorded at 240–360°C.*

**Table 2.**

*Results of electrical parameters of Na2CuP1.5As0.5O7.*

The values of the electrical parameters calculated at different temperatures are presented in **Table 2**.

The conductivity of Na2CuP1.5As0.5O7 (**Table 2**) increase from 0.35 105 S cm1 at 260°C to 3.13 105 S cm1 at 380°C. On the other hand, the porosity of 12% of our sample prompted us to estimate the conductivity values of the fully dense sample of Na2CuP1.5As0.5O7 using the empirical formula proposed by Langlois and Couret [30].

Taking into account the porosity factor P = 0.12, the conductivity value of the dense material will be σd = (4σ/0.88). The conductivity values of a dense sample, calculated at different temperatures, are summarized in **Table 2**. In this case, the experimental conductivity of 3.5 106 S cm1 corresponds to the corrected value of 1.59 105 S cm1 to 260°C.

The activation energy calculated from the slope of the linear curve Ln(σ T) = f(1000/T) is linear (**Figure 9**), with a slope that gives the value of Ea = 0.60 eV, satisfying Arrhenius law LnσT = Lnσ0 Ea/kT (k = Boltzmann constant).

The electrical study shows that the activation energy decreases for Na2CuP1.5As0.5O7 compared to that of Na2CuP2O7 [27], i.e., 0.60 eV and 0.89 eV, respectively. Therefore, the effect of P/As substitution increases the electrical conductivity of the parent phase Na2CuP2O7 at a lower temperature [31]. Overall, a comparison of the conductivity values of the material studied Na2CuP1.5As0.5O7 (at T = 350°C, σd = 88% = 2.28 10–5 S cm1 ; σd = 2.28 104 S cm1 and Ea = 0.60 eV) with those found in the literature shows that our material can be classified as fast ionic conductors.

**Figure 9.** *Arrhenius plot of conductivity of the Na2CuP1.5As0.5O7 sample.*

#### **3. Structure, magnetic, and magnetocaloric of La0.7Sr0.2K0.1MnO3 and La0.7Ba0.2Na0.1MnO3 compounds**

#### **3.1 Structural characterization**

Before proceeding with the magnetic study, it was necessary to ascertain the structure of the materials. Using the X-ray diffraction technique, it was possible to confirm that all the samples analyzed in this work have only one type of structure (single-phase) and that they corresponded to the given stoichiometry. The samples were reduced to powdered form, with a grinding time of 2 min, which was sufficient to obtain the random effect of orienting the structures without destroying them.

To check the nature and purity of the synthetic products, X-ray diffraction patterns were recorded at room temperature using the Panalytical X'Pert PRO diffractometer which is equipped with a copper anticathode (λKα1 = 1.54056 Å, λKα2 = 1.54439 Å) and provides good quality diffractograms.

**Figure 10** presents the experimental X-ray diffraction spectra of La0.7Sr0.2Na0.1MnO3 compound refined via the WinPlotr graphical interface of FullProf\_Suite which has been studied by W. Cheikh-Rouhou Koubaa et al. [32].

The Rietveld analysis of X-ray diffraction patterns shows that the La0.7Sr0.2K0.1MnO3 and La0.7Ba0.2Na0.1MnO3 compounds are single-phase and crystallize in the rhombohedral Th2Zn17-type structure (space group, R\_3m).

**Figure 11** shows the evolution of the unit cell volume compared to hrAi for the two series. For La0.7Sr0.2M0.1MnO3 samples, an observed increase of hrAi occurred with a slight decrease in the volume of the unit cell from 352.5 Å for *M* = Na to 351.1 Å for *M* = K. While in the compounds La0.7Ba0.2M0.1MnO3, the evolution of unit cells is rather governed by hrAi than by σ2 observed increase in unit cell volume. It should be noted that, although the ionic radius of site A and the size of the mismatch are similar for both samples, La0.7Sr0.2K0.1MnO3 and La0.7Ba0.2Na0.1MnO3, the unit cell volume has different values. This behavior highlights the effects of the difference in electronegativity between the A-site ions in our compounds [33]. With increasing hrAi, the MndOdMn binding angle decreases from 166.7° for sample La0.7Sr0.2Na0.1MnO3 to 165.9° for the sample La0.7Ba0.2Na0.1MnO3. However, the MndO bond length increases from 1.957 Å to

**Figure 10.** *X-ray diffraction patterns of La0.7Sr0.2Na0.1MnO3 sample.*

**Figure 11.** *Unit cell volume versus* h*rA*i *for La0.7M0.2M'0.1MnO3 (M = Sr, Ba and M' = Na, Ag and K) samples.*

1.968 Å, which influences the dual force of exchange. Effects of monovalent doping on the structural, magnetic, and magnetocaloric properties in La0.7*M*0.2*M*'0.1MnO3 manganese oxides (*M* = Sr, Ba and *M*' = Na, Ag, K) has been studied to control the value evaluation of magnetic entropy and cooling capacity.

#### **3.2 Study of magnetic properties**

The magnetic behaviors are subjected to two variables, which are the temperature and the applied magnetic field. It is possible to control these two factors experimentally and, thus, to keep one constant while studying the influence of the other on the magnetization M. The magnetic measurements were carried out on the two magnetometers BS2 and BS1 of the NEEL Institute used in the low/hightemperature configuration. The evolution of the magnetization as a function of the temperature and of the M(H, T) field was carried out on either side of the Curie temperature for each compound. The temperature of transition from the ferromagnetic state to the paramagnetic state was determined from the thermomagnetic curve M(T). TC corresponds to the minimum value of (dM/dT). The change in magnetic entropy was evaluated from the M(H, T) matrix based on the corresponding Maxwell equation.

Magnetic measurements of these compounds La0.7Sr0.2K0.1MnO3 and La0.7Ba0.2Na0.1MnO3 as a function of temperature in the temperature range 20–350 K under an applied magnetic field of 50mT show that all substituted samples show a magnetic transition from paramagnetic to ferromagnetic with decreasing temperature, as shown in **Figure 12**.

**Figure 13** shows that the Curie temperature varies according to the ionic rays. It goes to lower values with hrAi and varies from 340 K for hrAi = 1.182 Å to 311.5 K for hrAi = 1.247.

#### **3.3 Study of magnetic properties under an applied magnetic field**

In manganites, magnetism is essential of the localized type. A simple way to check, at low temperature, the ferromagnetic state, the fixed spin state, or the coexistence of a ferromagnetic and antiferromagnetic state of a sample, is to

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

**Figure 12.** *Temperature dependence of the magnetization under an applied magnetic field 50 mT.*

**Figure 13.** *Curie temperature versus* h*rA*i *for La0.7M0.2M'0.1MnO3 (M = Sr, Ba and M' = Na, Ag and K) samples.*

compare the value of the saturation magnetization with its calculated value assuming full alignment of the manganese spins of this compound. This procedure gives an estimate of the degree of alignment of the moments. For this, we present the study of the magnetic properties of these samples under an applied magnetic field up to the value of 7 tesla. As the study of the thermal variations of the magnetization under weak magnetic field (0.05 T) showed that the samples Pr0.6�xEuxSr0.4MnO3 (0.0 ≤ x ≤ 0.2) present a magnetic transition as a function of the temperature, it is then interesting to specify the nature of the magnetic order at low temperatures. M'nassri et al. [34] performed magnetization measurements as a function of the magnetic field applied at various temperatures.

The difference observed between the compounds La0.7Sr0.2K0.1MnO3 and La0.7Ba0.2Na0.1MnO3 is fully explained by the effects of the difference in electronegativity between the ions at site A for the two samples. The magnetization measurements as a function of the applied magnetic field up to 7 T at several temperatures confirmed the ferromagnetic behavior of these samples at low temperature, and the results are shown in **Figure 14**.

**Figure 14.** *(a) Isotherm magnetization curves M(H,T). (b) Temperature dependence of the spontaneous magnetization Msp and 1/χ for La0.7Sr0.2Na0.1MnO3 sample.*

The authors show that below TC, the magnetization M increases strongly with the magnetic field applied for H < 0.5 T then saturates above 1 T. The saturation magnet changes to higher values as the temperature decreases. This result confirms the ferromagnetic behavior of our sample at low temperature. **Figure 14b** shows the temperature dependence of the spontaneous magnetization Msp and 1/χ for the sample La0.7Sr0.2Na0.1MnO3. The experimental value of the spontaneous magnetization Msp(exp), deduced from the M(H) curves is 3.48 μB/Mn. The amplitude of the Msp(exp) is comparable to the theoretical value of 3.6 μB/mole calculated for full spin alignment. The critical exponent defined by.

$$\mathbf{Msp(T)} = \mathbf{Msp(0)} [\mathbf{1} - \mathbf{T}/\mathbf{T\_C}]^\nu \tag{1}$$

and deduced from the fit of the curve Msp(T) is 0.31, which confirms the ferromagnetic behavior of the samples from the group Cheikh-Rouhou team at low temperature

#### **3.4 Magnetocaloric effect**

Entropy is a measure of order in the magneto-thermodynamic system. High order is related to low entropy and vice versa. Dipoles, that is, electron spins, can take on different orientations. If these entities are oriented in the same direction in a paramagnetic material, a ferromagnetic or a diamagnetic material, the order and the magnetization are high. It is obvious that applying a magnetic field aligns the electronic spins and lowering the temperature (releasing energy from the system) also results in a more ordered arrangement. So the external magnetic field generates the stress parameter, while the magnetization determines the order parameter of such magnetic materials.

In order to obtain a maximum of information on the thermodynamic behavior and to observe the magnetocaloric effect in the manganites in the vicinity from

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

their magnetic transition temperature, we have studied the magnetocaloric behavior of the whole range of composition x.

After having studied magnetic measurements of magnetization as a function of temperature M(T) as well as magnetization as a function of the magnetic field at various temperature M(μ0H, T), we have calculated, the magnetic entropy change ΔSM of all our synthesized compounds as a function of temperature T and magnetic field H.

Based on the thermodynamic theory, magnetic entropy change is determined through the numerical integration of the magnetization isotherms, according to Maxwell's thermodynamic relation given by the following equation [34, 35]:

$$
\Delta \mathbb{S}\_{\mathcal{M}}(T, \Delta H) = \int\_0^H \mu\_0 \left(\frac{\partial \mathcal{M}}{\partial T}\right)\_{P,H} dH \tag{2}
$$

$$\Delta \mathbb{S}\_{\mathcal{M}}(T\_i, \Delta H) = -\sum\_{j} \frac{\mathcal{M}\_{i+1} \left(T\_{i+1}, H\_j\right) - \mathcal{M}\_i \left(T\_i, H\_j\right)}{T\_{i+1} - T\_i} \mu\_0 \delta H\_j \tag{3}$$

where Mi and Mi+1 are the magnetization values measured at the Hi field and at temperatures Ti and Ti+1, respectively [35].

We have shown in **Figure 15**, the variations in entropy (�ΔSM) as a function of temperature for different magnetic fields applied for these La0.7Sr0.2Na0.1MnO3 and La0.7*M*0.2*M*'0.1MnO3 sample (*M* = Sr, Ba and M' = Na, Ag and K) system. These curves show that the entropy value (�ΔSM) varies with temperature and has a peak around the transition temperature TC. The sample La0.7Sr0.2Na0.1MnO3 shows the highest value of ΔSM Max, 4.07 J kg�1 K�1 , around 345 K. For the sample La0.7Ba0.2Na0.1MnO3, we observed an asymmetric broadening of the peak ΔSM, which could be explained by a structural inhomogeneity. We can also notice that the magnetic and negative entropy for all our samples was due to the ferromagnetism encountered in these samples. The value of the entropy (�ΔSM Max) was also observed to increase when the applied magnetic field increased. Although these ΔSM Max values in these samples are lower than those observed in Gd or Gd-based compounds, the ΔSM Max curves as a function of temperature are significantly wider.

**Figure 15.** *Magnetic entropy change* �*ΔSM evolution versus temperature at (a) Several magnetic applied fields for La0.7Sr0.2Na0.1MnO3 sample (b) At 5 T for La0.7*M*0.2*M*'0.1MnO3 samples (*M *= Sr, Ba and* M*' = Na, Ag and K).*

This wider temperature range with a large change in magnetic entropy is useful for an ideal Ericsson refrigeration cycle. In addition, our samples are interesting in application as potential candidates in magnetic refrigeration because they are inexpensive, easier to manufacture, possess tunable TC, and have high chemical resistance stability.

### **4. Conclusion**

During our research on the investigation of new phosphates, arsenates of metals (Co, Cu) and alkali cations, we were able to isolate 2 crystalline phases. These compounds have shown remarkable structural diversity, even though they are isoformular. The synthesis method adopted was dry synthesis. In addition, P/As substitution showed conservation of structure in each material with variation in cation-oxygen distances. On the other hand, the effect of substitution is remarkable on the electrical properties. In fact, the sodium Co and Cu diphosphates-diarsenates are more conductive than the pure diphosphates. Alkaline atoms have an electronic procession, composed of a set of inert internal layers, having the structure of a rare gas, and an additional electron, or valence electron, which orbiting an s-type orbital. The magnetic and magnetocaloric study on the La0.7M0.2M'0.1MnO3 family shows that there is a Curie temperature in the vicinity of the room temperature and very high magnetic entropy values. We can conclude that these compounds are good candidates for magnetic refrigeration.

### **Author details**

Amira Marzouki1 , Ameni Brahmia2,3, Riadh Marzouki2,4\*, Mosbah Jemmali4 , Ismat H. Ali2 and Mohamed Faouzi Zid5

1 Laboratory of Signal Image and Energy Mastery, Engineering National Higher School of Tunis, Tunis, Tunisia

2 Chemistry Department, College of Science, King Khalid University, Abha, Saudi Arabia

3 Laboratoire des Materiaux et de l'Environnement pour le Developpement Durable, LR18ES10, University of Tunis El Manar, Tunisia

4 Faculty of Science, LSME, University of Sfax, Sfax, Tunisia

5 Faculty of Sciences of Tunis, Laboratory of Materials, Crystallochemistry and Applied Thermodynamics, University of Tunis El Manar, Tunisia

\*Address all correspondence to<EMAIL_ADDRESS>
© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

#### **References**

[1] Brian LE, Nazar F. Sodium and sodium-ion energy storage batteries. Current Opinion in Solid State & Materials Science*.* 2012;**16**:168-177

[2] Shakoor RA, Seo DH, Kim H, Park YU, Kim J, Kim SW, et al. A combined first principles and experimental study on Na3V2(PO4)2F3 for rechargeable Na batteries. Journal of Materials Chemistry. 2012;**22**:20535

[3] Kim H, Shakoor RA, Park C, Yeon Lim S, Kim JS, Jo YN, et al. Na2FeP2O7 as a positive electrode material for rechargeable aqueous sodium-ion batteries. Advanced Functional Materials. 2013;**23**:1147

[4] Peng Y, Yang L, Ju X, Liao B, Ye K, Li L, et al. A comprehensive investigation on the thermal and toxic hazards of large format lithium-ion batteries with LiFePO4 cathode. Journal of Hazardous Materials. 2020;**381**:120916

[5] Goodenough JB, Hong HY-P, Kafalas JA. Fast Na+ -ion transport in skeleton structures. Materials Research Bulletin. 1976;**11**:203-220

[6] Kanazawa T. Inorganic Phosphate Materials, Materials Science Monographs. Vol. 52. Amsterdam: Elsevier; 1989

[7] Erragh F, Boukhari A, Elouadi B, Holt EM. Crystal structures of two allotropic forms of Na2CoP2O7. Journal of Crystallographic and Spectroscopic Research. 1991;**21**:321-326

[8] Padhi AK, Nanjundaswamy K, Goodenough JB. Phospho‐olivines as positive‐electrode materials for rechargeable lithium batteries. Journal of the Electrochemical Society. 1997; **144**(4):1188-1194

[9] Durif A. Crystal Chemistry of Condensed Phosphates. US: Springer; 1995 [10] Barpanda P, Lu J, Ye T, Kajiyama M, Chung S-C, Yabuuchi N, et al. A layerstructured Na2CoP2O7 pyrophosphate cathode for sodium-ion batteries. RSC Advances. 2013;**3**: 3857-3860. DOI: 10.1039/C3RA23026K

[11] Satya Kishore MVVM, Varadaraju UV. Synthesis, characterization and electrochemical studies on LiCoAsO4. Materials Research Bulletin. 2006;**41**: 601-607

[12] Essehli R, Ben Yahia H, Maher K, Sougrati MT, Abouimrane A, Park J-B, et al. Unveiling the sodium intercalation properties in Na1.86□0.14Fe3(PO4)3. Journal of Power Sources. 2016;**324**: 657-664

[13] Tishin AM, Spichkin YI. The Magnetocaloric Effect and Its Applications. Bristol: Institute of Physics Publishing; 2003

[14] Tegus O, Brück E, Buschow KHJ, de Boer FR. Transition-metal-based magnetic refrigerants for roomtemperature applications. Nature. 2002; **415**:150

[15] Cherif K, Dhahri J, Dhahri E, Oumezzine M, Vincent H. Effect of indium substitution on structural, magnetic and magnetocaloric properties of La0.5Sm0.1Sr0.4Mn1xInxO3 (0 ≤ x ≤ 0.1) manganites. Journal of Solid State Chemistry. 2002;**163**:466

[16] Tlili MT, Chihaoui N, Bejar M, Dhahri E, Valente MA, Hlil EK. Charge ordering analysis by electrical and dielectric measurements in Ca2xPrxMnO4 (x = 0–0.2) compounds. Journal of Alloys and Compounds. 2011; **509**:6447

[17] Daoudi A, Le Flem G. Sur une série de solutions solides de formule Ca2xLnxMnO4 (Ln = Pr, Nd, Sm, Eu,

Gd). Journal of Solid State Chemistry. 1972;**5**:57

[18] Takahashi J, Kamegashira N. Lowtemperature structural phase transitions in rare earths substituted calcium manganese oxides [Ca2�xLnxMnO4, where Ln=Nd, Sm-Lu and Y]. Materials Research Bulletin. 1993;**28**:451

[19] Takahashi J, Kikuchi T, Satoh H, Kamegashira N. Phase transition of Ca2-xSmxMnO4 (x < 0.5). Journal of Alloys and Compounds. 1993;**192**:96

[20] Dhahri E, Guidara K, Cheikhrouhou A, Joubert JC, Pierre J. Monovalent effects on structural, magnetic and magnetoresistance properties in doped manganite oxides. Phase Transitions. 1998;**66**:99

[21] Teresa JMDE, Ibarra MR, Garcia J, Blasco J, Ritter C, Algarabel PA, et al. Spin-glass insulator state in (Tb-La)2/3Ca1/3MnO3 perovskite. Physical Review Letters. 1996;**76**:3392

[22] Ju HL, Kwon C, Li Q, Greene RL, Venkatesan T. Magnetic anisotropy and strain states of (001) and (110) colossal magnetoresistance thin films. Applied Physics Letters. 1994;**65**:2108

[23] Ziese M. Extrinsic magnetotransport phenomena in ferromagnetic oxides. Reports on Progress in Physics. 2002; **65**:143

[24] Shankar KS, Kar S, Raychaudhuri AK, Subbanna GN. Fabrication of ordered array of nanowires of La0.67Ca0.33MnO3La0.67Ca0.33MnO3 (x=0.33)(x=0.33) in alumina templates with enhanced ferromagnetic transition temperature. Applied Physics Letters. 2003;**84**(6):993

[25] Larson AC, Von Dreele RB. General Structure Analysis System (GSAS). Report LAUR 86–748. Los Alamos, NM: Los Alamos National Laboratory; 2000

[26] Marzouki R, Ben Smida Y, Sonni M, Avdeev M, Zid MF. Synthesis, structure, electrical properties and Na+ migration pathways of Na2CoP1.5As0.5O7. Journal of Solid State Chemistry. 2020;**285**: 121058

[27] ALQarni OSA, Marzouki R, Smida YB, Avdeev M, Alghamdi MM, Zid MF. Synthesis, electrical properties and Na+ migration pathways of Na2CuP1.5As0.5O7. PRO. 2020;**8**:305

[28] MacDonald JR. Impedance Spectroscopy. New York: Wiley; 1987

[29] Sanz F, Parada C, Rojo JM, Ruiz-Valero C, Saez-Puche R. Studies on tetragonal Na2CoP2O7, a novel ionic conductor. Journal of Solid State Chemistry. 1999;**145**(2):604-611. DOI: 10.1006/jssc.1999.8249

[30] Langlois S, Couret F. Electrochemical measurements of mass transfer coefficients in a cell simulating tooth canals. Journal of Applied Electrochemistry. 1989;**19**:43

[31] Hafidi E, El Omari M, El Omari M, Bentayeb A, Bennazha J, El Maadi A, et al. Conductivity studies of some diphosphates with the general formula AI 2BIIP2O7 by impedance spectroscopy. Arabian Journal of Chemistry. 2013;**6**: 253-263

[32] Cheikh-Rouhou Koubaaa W, Koubaaa M, Cheikhrouhou A. Effect of monovalent doping on the structural, magnetic and magnetocaloric properties in La0.7M0.2M0 0.1MnO3 manganese oxides (M=Sr, Ba and M0 =Na, Ag, K). Physics Procedia. 2009;**2**:989-996

[33] Koubaa WC, Koubaa M, Cheikhrouhou A. Structural, magnetotransport, and magnetocaloric properties of La0.7Sr0.3�xAgxMnO3 perovskite manganites. Journal of Alloys and Compounds. 2008;**453**:42

*Correlation between Structure, Electrical, and Magnetic Properties of Some Alkali-Oxide… DOI: http://dx.doi.org/10.5772/intechopen.102322*

[34] M'nassri R, Cheikhrouhou-Koubaa W, Koubaa M, Boudjada N, Cheikhrouhou A. Magnetic and magnetocaloric properties of Pr0.6-xEuxSr0.4MnO3 manganese oxides. Solid State Communications. 2011; **151**:1579

[35] Mc Michael RD, Ritter JJ, Shull RD. Enhanced magnetocaloric effect in Gd3Ga5xFexO12. Journal of Applied Physics. 1993;**73**:6946

### *Edited by Riadh Marzouki*

Alkaline elements are present in large quantities and in different forms in the Earth's layers. They are widely used in the manufacture of materials showing interesting physical properties that can be applied in several fields, including catalysis, biology, energy, and others. This book describes different methods of synthesis and treatment of certain alkaline materials and their applications in different fields. It discusses alkaline chemistry in catalysis, biology, polymers and composites, and crystallography.

Published in London, UK © 2022 IntechOpen © SimoneN / iStock

Alkaline Chemistry and Applications

Alkaline Chemistry and

Applications

*Edited by Riadh Marzouki*


**Abstract**

During the last decade, cellulose structural features have been revisited, with particular focus on its structural anisotropy (amphiphilicity) and interactions determining its recalcitrance to dissolution. Evidences for cellulose amphiphilicity are patent, for instance, in its capacity to adsorb at oil–water interfaces, thus being capable of stabilizing emulsions. This behavior is observable in all its forms, from cellulose nanoparticles to macromolecules. This chapter is divided into two main parts; first, the fundamentals of emulsion formation and stabilization will be introduced, particularly focusing on the role of natural emulsifiers. Secondly, the emerging role of cellulose as a natural emulsifier, where the ability of cellulose to form and stabilize emulsions is revisited, from cellulose nanoparticles (Pickering-like effect) to macromolecules (i.e., cellulose derivatives and native molecular cellulose).

**Keywords:** Nanocelluloses, regenerated cellulose, cellulose derivatives, native molecular cellulose, amphiphilicity, oil–water interface, emulsifiers, adsorption, emulsions, stabilization


*Review* **New Imaging Parameters to Predict Sudden Cardiac Death in Chagas Disease**

**Renata J. Moll-Bernardes 1, Paulo Henrique Rosado-de-Castro 1,2, Gabriel Cordeiro Camargo 1, Fernanda Souza Nogueira Sardinha Mendes 3, Adriana S. X. Brito 1 and Andréa Silvestre Sousa 1,2,3,\***

Received: 7 April 2020; Accepted: 6 May 2020; Published: 8 May 2020

**Abstract:** Chronic Chagas' cardiomyopathy is the most severe and frequent manifestation of Chagas disease, and has a high social and economic burden. New imaging modalities, such as strain echocardiography, nuclear medicine, computed tomography and cardiac magnetic resonance imaging, may detect the presence of myocardial fibrosis, inflammation or sympathetic denervation, three conditions associated with risk of sudden death, providing additional diagnostic and/or prognostic information. Unfortunately, despite its high mortality, there is no clear recommendation for early cardioverter-defibrillator implantation in patients with Chagas heart disease in the current guidelines. Ideally, the risk of sudden cardiac death may be evaluated in earlier stages of the disease using new image methods to allow the implementation of primary preventive strategies.

**Keywords:** Chagas disease; cardiomyopathy; myocardial fibrosis; myocardial sympathetic denervation; inflammation; sudden death; cardiac magnetic resonance; radionuclide imaging; SPECT-CT; PET-CT

#### **1. Introduction**

Chronic Chagas' cardiomyopathy is the most severe and frequent manifestation of Chagas disease and occurs in 25–30% of infected people. Patients may develop severe clinic manifestations, such as congestive heart failure, malignant arrhythmias or thromboembolism [1,2].

The disease is endemic in Latin America, where it is responsible for a high social and economic burden, however, due to the migration of affected individuals and the increased number of patients infected through other forms of transmission, such as blood transfusion, organ transplantation or vertically from mother to infant, the disease has become a global health concern. Non-endemic countries have reported increasing numbers of patients and are particularly worried about the limited awareness in medical community [3]. Cardiac involvement is the main cause of death. The clinical course of Chagas heart disease (CHD) is variable, and the identification of patients at risk for death remains a challenge [4].

The pathogenesis of chronic CHD involves a progressive inflammatory process and adverse immune response with myocardial necrosis and damage to the conduction tissue, leading to electrocardiographic changes, such as atrioventricular and intraventricular blocks, sinus node dysfunction, ventricular arrhythmias and sudden cardiac death. The presence of myocardial inflammation, necrosis and fibrosis may result in left ventricular (LV) segmental wall motion abnormalities and congestive heart failure. Derangements in the coronary microcirculation are likely to cause ischemic-like symptoms, electrocardiographic (ECG) abnormalities and perfusion defects [5].

Patients with CHD are classified according to symptoms and presence of myocardial abnormalities. The presence of typical ECG abnormalities usually defines the diagnosis of chronic CHD (Table 1). The most common initial findings are conduction disorders, and/or ventricular arrhythmias [6].

**Table 1.** Definition and progression of chronic Chagas heart disease.

Adapted from: Xavier SS et al. [7]. LVEF = left ventricle ejection fraction.

Chronic CHD carries a worse prognosis when compared to ischemic and other non-ischemic causes of heart failure [8]. The presence of chronic persistent myocardial inflammation plays a central role in the genesis of arrhythmias, due to irreversible cell damage, fibrosis and scar formation. Ventricular arrhythmias are a major cause of morbidity and mortality in patients with Chagas disease, and may occur even before significant LV systolic dysfunction, leading to sudden cardiac death (SCD) [9,10]. In addition, active inflammation may increase the automaticity within inflamed areas and act as a trigger for reentry in the presence of fibrosis. Besides fibrosis and inflammation, autonomic dysfunction can also be related to the genesis of ventricular arrhythmias.

Identifying patients with chronic CHD at increased risk of SCD is crucial, as they could benefit from prophylactic implantation of cardioverter-defibrillators [11–13]. ECG, 24 h Holter and exercise test are extremely useful to diagnose and manage these patients. In addition, new imaging modalities such as strain echocardiography, nuclear medicine and cardiac magnetic resonance (CMR) imaging with late gadolinium contrast enhancement may detect the presence of myocardial fibrosis, inflammation or sympathetic denervation, providing additional diagnostic and/or prognostic information. Acute Chagas disease will not be analyzed here.

#### **2. ECG**/**Holter Monitoring**/**Exercise Test**

#### *2.1. Eletrocardiogram (ECG)*

Electrocardiogram (ECG) is the most important exam to characterize in which phase Chagas disease patient is. It is an easy method to perform, widely available worldwide and with high sensitivity. If the ECG remains normal, the prognosis of those patients is similar to the general population. It is important to perform ECG on a serial basis annually to assess possible disease progression. Asymptomatic infected patients with normal ECG (chronic indeterminate form) should be followed annually with clinical evaluation and ECG. In the presence of symptoms and/or ECG abnormalities, patients should be ideally submitted to 24-h Holter, exercise test and echocardiography. Usually, the ECG is the first exam to undergo changes when there is cardiac involvement in the chronic phase of Chagas disease. The presence of typical ECG abnormalities is associated with increased risk of progression to more severe cardiomyopathy.

There are some unspecific findings in ECG that need individualized evaluation, as they alone are not sufficient to diagnose cardiac involvement such as: sinus bradycardia, low voltage QRS, incomplete right bundle branch block, anterosuperior hemiblock, first-degree atrioventricular block and unspecific ST-T findings. The most commons early findings are right bundle branch block with or without left anterior fascicular block, second- and third-degree atrioventricular block. Other alterations associated with CHD are complex ventricular arrythmias, atrial fibrillation, complete

atrioventricular block and left brunch block. ECG changes associated with worse prognosis are frequent premature ventricular contractions (PVCs), increased QT-interval dispersion, low-voltage QRS, QRS fragmentation, polymorphous or repetitive non-sustained ventricular tachycardia and prolonged QRS.

When an electrocardiographic alteration compatible with Chagas' disease is identified, the patient is reclassified from chronic undetermined benign form to a cardiac form with a poorer prognosis and additional cardiac and gastrointestinal check-up is recommended (Table 1) [6].

#### *2.2. 24 h Holter*

A 24 h Holter is recommended when available and in patients with symptoms suggestive of cardiac arrhythmias, such as palpitations, presyncope and syncope, or ECG abnormalities like bradyarrhythmia, second degree atrioventricular block and multiple PVCs. Often, brady and tachyarrhythmias are identified in the same patient, and this differential diagnosis is important for indicating a pacemaker implant associated or not with the implantable cardioverter defibrillator (ICD). A 24 h Holter is useful to identify an increased risk of sudden cardiac death and unmask signs of autonomic dysfunction, such as reduced heart rate variability. The presence of non-sustained ventricular tachycardia on the 24 h Holter is one criterion that worsens the prognosis of patients with CHD. Major electrocardiographic changes are sinus node dysfunction, atrioventricular block, or frequent PVCs that becomes more complex and frequent with the disease progression. The presence of ventricular tachycardia is associated with left or global ventricular dysfunction and, when sustained, configures a worse prognosis. The main cause of sudden death in CHD is ventricular fibrillation, which is more frequent when there are previous episodes of ventricular tachycardia, but it can be the first manifestation of the disease or its terminal event in patients with severe ventricular dysfunction and heart failure. CHD is one of the most common indications of artificial pacemaker implants in Brazil, and a 24 h Holter is important to help the clinician to confirm the need of the device, indicate the best type and the follow-up after the implant, as this can detect faulty functioning of the stimulation system or orientate better programming [14]. Regarding the autonomic function, a 24 h Holter can assist using spectral analysis. CHD is associated with a reduction in the sympathetic response (reduced LF—low frequency), as well as an overall decrease in autonomic function observed by the reduction of Standard deviation of sequential 5-min N-N interval means (SDANN). The increased rMSSD (square root of the mean of the square of the differences between adjacent normal RR intervals, in a time interval) and pNN50 (percentage of absolute differences in successive normal sinus values >50 ms) values reflect malfunction in their vagal tone.

#### *2.3. Exercise Test*

When available, exercise tests should be performed to understand the responses of Chagas disease patients to hemodynamic stress and to assess the functional capacity. The most important finding to observe on ECG during the exam are ventricular arrhythmias, which are independent predictors of mortality [1,15]. They indicate progression of CHD and worsening of cardiomyopathy. The chronotropic response and the heart rate in the first minute of recovery can show how sympathetic and parasympathetic act, since there is an autonomic dysfunction in Chagas disease patients. The assessment of functional capacity is important to guide professional and activity restrictions, to follow the worsening on function capacity and beginning of heart failure. In advanced heart failure patients, the exercise test can suggest a cardiac transplantation indication when the peak oxygen uptake ≤10 mL/kg/min. Finally, exercise test could also useful to access ST changes in differential diagnosis in patients with classical chest pain, to exclude coronary disease and to elucidate patient's symptoms during exercise.

#### **3. Echocardiography**

Echocardiography has become the most common method to assess and follow up patients with CHD. The presence of echocardiographic abnormalities allows us to stage disease progression (Table 1). In early stages, echocardiography may demonstrate segmental wall motion abnormalities, particularly

in the apex or basal segments of inferior and inferolateral walls, usually without associated obstruction in epicardial coronary arteries [16]. These lesions usually occur due to microcirculatory derangements. Diastolic dysfunction is also a common early finding.

The landmark lesions of CHD are LV apical aneurysms. In order to identify these aneurysms, it is important to use not only standard but also angulated views such as modified four- and two-chambers views, aiming posteriorly and avoiding foreshortening, dropout or near-field artifacts. Aneurysms are not limited to the apex or inferolateral walls, the most common sites, and may be associated to the presence of intraventricular mural thrombi. Right ventricular (RV) aneurysm is uncommon. The prevalence of LV aneurysm is inferior to 10% in asymptomatic patients but has been reported as superior to 50% in patients with more advanced stages, with moderate to severe LV global systolic dysfunction. Symptomatic patients may present regional wall motion abnormalities, LV or biventricular dilatation with diminished LV ejection fraction.

Echocardiograms should be performed at least once in every patient with positive serology and can be repeated each 3–5 years or anytime if ECG abnormalities are detected and should follow the European Association of Cardiovascular Imaging/American Society of Echocardiography task force's recommendations on chamber quantification, LV function and morphology, RV function and valvular analysis [17].

Chamber volumes and global ventricular function should be evaluated by bidimensional (2D) echocardiography through biplane method of disks (Simpson's) and tridimensional (3D) echocardiography, whenever possible. Tridimensional is more accurate than 2D echo for assessing LV volumes and ejection fraction (EF), particularly in the presence of wall motion abnormalities when there is a distorted LV geometry, as it allows visualization of cardiac chambers without geometric assumptions. It can also avoid foreshortening, being more accurate to detect the presence of aneurysms or thrombus. Regional wall motion abnormalities are most often located in the apex, inferior and inferolateral walls and should be assessed in multiple angles. Evaluation should include valvular structure and function, as functional mitral and tricuspid valve regurgitation are common in advanced cases with ventricular remodeling and valve annular dilation.

Strain is a measure of myocardial deformation, defined as the change of length of the myocardium that allows a more precise and quantitative measurement of the regional myocardial function, overcoming the subjective evaluation by conventional echocardiography. Strain obtained by speckle-tracking is the method of choice because it is not angle dependent. Regional strain is of particular interest in CHD due to the frequent segmental myocardial involvement (Figure 1), being particularly useful for increased recognition of subclinical myocardial dysfunction during indeterminate form of CHD [18,19]. Global longitudinal strain is the most validated method and is correlated with the amount of myocardial fibrosis, as shown is a study including 125 patients with Chagas disease, which found that longitudinal strain was reduced in the patients who had cardiac fibrosis on CMR, despite no significant difference in LV ejection fraction compared with patients without cardiac fibrosis [20].

Echocardiography may potentially be used to study ventricular dyssynchrony in CHD [21,22]. Duarte et al. [22] reported that the prevalence of interventricular dyssynchrony was 34% and of intraventricular dyssynchrony was 85%. However, dyssynchrony was not a strong predictor of clinical events, and the ECG remains the most important tool for indicating cardiac resynchronization therapy. Contrast echocardiography can be useful to enhance LV endocardial border and increase detection of aneurysms and thrombus [15].

Echocardiography is an inexpensive and readily available method, being very useful to stage and follow these patients, giving hemodynamic information that can be extremely helpful in advanced heart failure. On the other hand, the method is less sensitive to evaluate the presence of fibrosis and is not able to detect inflammation.

**Figure 1.** Global longitudinal strain of the left ventricle and strain curves depicting increased mechanical dispersion. Abnormal findings of left ventricular (LV) longitudinal strain in apical three-chamber view (**top left**); apical two-chamber view (**bottom left**); apical four-chamber view (**bottom right**); "Bull's-eye" plot of strain values for each myocardial segment evidences anteroseptal and inferolateral akinesia represented by blue areas in the polar map (**top right**).

#### **4. Computed Tomography**

Cardiac computed tomography (CT) has an excellent negative predictive value to exclude coronary artery disease through coronary computed tomography angiogram (CTA), being extremely useful in patients with low to intermediate pre-test probability. Cardiac CT can also be used to plan electrophysiologic procedures, evaluate LV morphology, including the detection of regional wall motion abnormalities, apical aneurysms and intracardiac thrombi, and to calculate LV function in patients with difficult echocardiographic windows and contraindication to CMR, such as the presence of an implanted cardiac device [1,5].

#### **5. Cardiovascular Magnetic Resonance**

CMR has a superior capability for anatomic and functional evaluation of cardiac chambers and measurement of right and left ventricular EF. LV systolic dysfunction is the strongest predictor of morbidity and mortality in CHD, and detection of subclinical dysfunction may orient therapeutic measures that would help in delaying the progression of the disease. Regional wall motion abnormalities, thrombus and aneurysms can be easily recognized (Figure 2).

**Figure 2.** Cardiac magnetic resonance (CMR) four chamber image showing a typical left ventricular mamillar apical aneurysm (arrow). CMR: cardiac magnetic resonance.

Myocardial fibrosis is a histopathological finding common to several types of cardiovascular diseases and is associated with morbidity and mortality [23,24]. The mechanisms leading to fibrosis are variable and there may be an imbalance between collagen production and degradation or myocyte death [25].

The presence of fibrotic myocardial lesions is associated to reentrant circuits, which are the main pathophysiologic mechanism of ventricular tachyarrhythmias. Recent studies reveal that the degree of myocardial fibrosis increases progressively from the mildest to the most severe disease stages and late gadolinium contrast enhancement (LGE) evaluated on CMR is currently the best non-invasive method to assess it, being a marker of subclinical involvement with proven prognostic value [26–28]. The presence of scar by LGE is strongly associated with high risk of SCD [29].

Development of fibrosis in Chagas disease patients predominates in territories of distal circulation, particularly in apical and inferolateral regions, and the pattern of LGE is usually meso-epicardial or transmural (Figure 3), which differs from ischemic cardiomyopathies, in which LGE with a characteristic subendocardial fibrosis pattern matches a coronary territory distribution [11].

**Figure 3.** CMR images in four chamber (**A**) and three chamber (**B**) views showing meso-epicardial (arrows) and apical transmural (arrowheads) late gadolinium enhancement (LGE). CMR: cardiac magnetic resonance; LGE: late gadolinium enhancement.

Nonetheless, LGE fails to account for interstitial and diffuse collagen distribution, which leads to underestimation of the total myocardial fibrosis mass [30]. The capability of tissue characterization with detection of myocardial edema and interstitial fibrosis has been recently reported in ischemic and non-ischemic cardiomyopathies and may be helpful in risk assessment [31]. New CMR techniques, such as native T1 mapping (Figure 4) and myocardial extracellular volume (ECV) calculation quantify diffuse myocardial fibrosis with close correlation with histological studies [32]. Avanesov et al. [31] reported that global ECV was superior to other CMR parameters, including LGE, to identify hypertrophic cardiomyopathy patients with syncope and non-sustained ventricular tachycardia, and presumably with increased risk of SCD. Future studies are necessary to evaluate the usefulness of these techniques in Chagas disease.

**Figure 4.** Four chamber image with color-coded native (non-contrast) T1 map, indicating the presence of mesocardial septal fibrosis (arrow) and apical transmural fibrosis (arrowheads). Increased native T1 is represented by yellow areas within the myocardium.

#### **6. Nuclear Medicine**

Patients with chronic CHD have an increased risk of SCD compared to other cardiomyopathies. The presence of fibrosis, dysautonomia and persistent cardiac inflammation could contribute to the risk of SCD, offering a substrate for ventricular tachycardia (VT). Molecular imaging with single photon emission computed tomography (SPECT) or positron emission tomography (PET) using a variety of radiotracers are valuable tools to identify changes that predispose to arrhythmia, such as hypoperfusion, inflammation and abnormal sympathetic innervation.

Myocardial perfusion imaging may be performed with SPECT or hybrid SPECT-CT equipment, using technetium-99m labeled radiopharmaceuticals such as tetrofosmin or sestamibi (Figure 5), or with PET and hybrid PET-CT using radiopharmaceuticals such as Rubidium-82. A disturbance due to microvascular ischemia participates in the mechanisms, causing myocardial injury and can occur in the early stages of CHD [33]. The use of hybrid SPECT-CT and PET-CT equipment allows attenuation correction, reducing artifacts.

**Figure 5.** Automatic quantification (polar map) from rest 99m-Technetium sestamibi scintigraphy demonstrating reduced myocardial perfusion in apical region in a patient with a small aneurism of the left ventricular apex.

It has been demonstrated that histogram bandwidth and phase-derived standard deviation may be analyzed in gated-SPECT myocardial perfusion imaging (Figure 6), which allows the assessment of intraventricular synchronism. In comparison to echocardiography, nuclear medicine techniques have the advantage of lower interobserver variability and higher reproducibility. Peix et al. [34] studied myocardial perfusion and intraventricular synchronism in the indeterminate phase of Chagas disease which presented tissue Doppler imaging-derived strain. A total of 8% of subjects had perfusion defects, 28% had a post-stress LVEF reduction of >5%. The authors found that histogram bandwidth and phase-derived standard deviation indicated a significant difference between post-stress and rest. In these cases, there was a minor dyssynchrony at rest that normalized at post-stress.

Abnormalities in cardiac sympathetic innervation may play a central role in the mechanism triggering serious ventricular arrhythmias and SCD in patients with non-ischemic cardiomyopathy. The work of Miranda [35] showed that regional myocardial sympathetic denervation assessed with Iodine-123 metaiodobenzylguanidine (123I-MIBG) scintigraphy is associated with sustained ventricular tachycardia in CHD, concluding that viable although denervated myocardial areas were associated to the genesis of sustained ventricular arrhythmias (Figure 7). Cardiac autonomic sympathetic modulation detected with 123I-MIBG SPECT can also be affected in subjects with early forms of CHD with preserved ventricular function, as reported by Landesmann [36]. Recently, Gadioli [37] used 123I-MIBG SPECT to assess the extension of the sympathetic denervation, and showed its association with the severity of the ventricular arrhythmias. It is necessary to design prospective studies for the assessment of cardiac sympathetic innervation. Results could be used for risk stratifications of sudden death, which

would be extremely useful if we consider the lack of consensus related to the stratification and primary prevention of sudden cardiac death in patients with Chagas disease.

**Figure 6.** Multigated acquisition (MUGA) scan demonstrating reduced ejection fraction (EF = 21%) in a patient with Chagasic cardiomyopathy. This radionuclide ventriculography technique is a highly accurate test, used to determine the heart's pumping function, and it shows substantial reproducibility and low intraobserver and interobserver variability.

**Figure 7.** A 123I-MIBG scintigraphy planar scintigraphy in anterior view at 3 h (**A**) to evaluate the sympathetic innervation a patient with Chagasic cardiomyopathy and ventricular arrhythmia. Heart to mediastinum ratio at 3 h was reduced and radiotracer washout was increased. Single photon emission computed tomography (SPECT)-computed tomography (CT) images in coronal (**B**), sagittal (**C**) and transversal (**D**) planes demonstrated reduced cardiac uptake of 123I-MIBG in apical, inferior and lateral regions indicating sympathetic denervation in these areas and a worse prognosis.

The presence of persistent cardiac inflammation may also be related to ventricular tachycardia and the risk of sudden death. Persistent subclinical inflammation in areas adjacent to fibrotic regions may act as a trigger for reentry in the presence of fibrosis. In addition, active inflammation may increase the automaticity within inflamed areas. Radionuclide imaging can allow the identification of areas of inflammation of the myocardium in patients with non-ischemic cardiomyopathies [38].

18F-fluorodeoxyglucose (18F-FDG) is a glucose analogue being physiologically captured by cardiomyocytes which can be used to evaluate the pathologic uptake by inflammatory cells. Recently, the use of 18F-FDG PET-CT has been studied in patients with sarcoidosis, demonstrating that the presence of focal perfusion defects and increased FDG uptake in patients with suspected cardiac sarcoidosis identified subjects at higher risk of death or ventricular tachycardia [39].

The use of PET-CT in Chagas disease has not yet been systematically studied. There are several similarities between Chagas disease and sarcoidosis, so the use of this imaging technique could potentially be useful in assessing the presence of inflammation and prognosis. Both are associated with changes in the conduction system and complex ventricular arrhythmias and with the presence of meso-epicardial fibrosis.

Currently, patients with chronic Chagas disease are not routinely evaluated for the presence of inflammation. Three case reports have described increased uptake of 18F-FDG PET in patients with Chagas disease and ventricular tachycardia, suggesting that the presence of inflammation could be involved in the genesis of the arrhythmia [40,41]. Recently, the use of new radiotracers that are not physiologically accumulated in the normal myocardium, including somatostatin receptor imaging radiopharmaceuticals such as gallium-68 (68Ga) labeled-DOTATOC may overcome some limitations of the 18F-FDG, as has been reported for cardiac sarcoidosis and CHD [42,43], as illustrated in Figure 8.

**Figure 8.** Increased cardiac volume in a patient with Chagasic cardiomyopathy. Mild increase in uptake of 18F-FDG (**A**,**B**) and of 68Ga-DOTATOC (**C**,**D**) in positron emission tomography (PET)-CT images was restricted to the basal anterolateral segment, indicating the presence of inflammation in this segment.

PET-CT has the advantage of a higher spatial resolution when compared to scintigraphy and, unlike CMR, it can be used in patients with implantable cardiac devices. Better understanding of the role of inflammation in these patients may provide novel treatment strategies, such as localization of the anatomic substrate before ablation of VT, and improved risk stratification and orientation about primary prevention with ICD implantation.

Unfortunately, despite its significant mortality, there is no clear recommendation for early ICD implantation in patients with CHD in the current guidelines. Ideally, the risk of SCD should be evaluated in earlier stages of the disease. New imaging parameters that are useful to identify factors involved in the genesis of arrhythmia such as fibrosis, inflammation and dysautonomia (Figure 9) can be a promising strategy.

#### **7. Conclusions**

Fibrosis, inflammation and dysautonomia are involved in the genesis of ventricular arrhythmia in CHD. Due to the risk of sudden death, it is important that such events are early identified, so that SCD preventive measures can be taken. Myocardial fibrotic areas leads to a macro reentry circuit that triggers ventricular tachycardia and CMR is an accurate method to identify these areas. The presence of inflammatory process, detected through PET-CT with FDG or DOTATOC, may be involved in the genesis of the arrhythmia in CHD. Finally, studies of myocardial dysautonomia identified using myocardial SPECT-CT with MIBG have reported an association between denervated areas and ventricular electrical instability, even in the presence of viable myocardium. Echocardiography, despite not being sensitive enough to identify fibrosis and denervation, can indicate severe ventricular dysfunction, which is also associated with an increased risk of sudden death. Other non-invasive tests such as ECG, 24 h Holter and exercise test can identify both dysautonomia, and the presence of ventricular electrical instability, demonstrating an increased risk of sudden death in CHD patients. Thereby, traditional non-invasive diagnostic methods associated to new image parameters obtained through CMR, SPECT-CT and PET-CT can improve risk stratification of SCD and play a promising role in the implementation of primary preventive strategies.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).