- OLDHAMITE-NININGERITE SERIES
- Mg7CaS8

This is a term of the MgS-Cas solid solution with ordered super-structure: every 8th Mg atom is replaced by 1 Ca. This ordering breaks the symmetry and makes some of the former IR modes of the ideal disordered (Mg,Ca)S solid solution Raman-acive. See article for details and experimental measurements.

Crystal Structure

Because of the translational symmetry all the calculations are performed in the primitive unit cell and not in the conventional unit cell. The following information regarding the structure is given with respect to this primitive unit cell, which sometimes can take an unintuitive shape.

Symmetry (experimental):

Space group:

225

Fm-3m

Lattice parameters (Å):

Angles (°):

Symmetry (theoretical):

Space group:

225

Fm-3m

Lattice parameters (Å):

10.3749

10.3749

10.3749

Angles (°):

60

60

60

Cell contents:

Number of atoms:

16

Number of atom types:

3

Chemical composition:

0

Atomic positions (theoretical):

Ca:

0.0000

0.0000

0.0000

Mg:

0.5000

0.0000

0.0000

Mg:

0.0000

0.5000

0.0000

Mg:

0.5000

0.5000

0.0000

Mg:

0.0000

0.0000

0.5000

Mg:

0.5000

0.0000

0.5000

Mg:

0.0000

0.5000

0.5000

Mg:

0.5000

0.5000

0.5000

S:

0.2500

0.2500

0.2500

S:

0.7412

0.2588

0.2588

S:

0.2588

0.7412

0.2588

S:

0.7412

0.7412

0.2588

S:

0.2588

0.2588

0.7412

S:

0.7412

0.2588

0.7412

S:

0.2588

0.7412

0.7412

S:

0.7500

0.7500

0.7500

Atom type

X

Y

Z

We have listed here the reduced coordinates of all the atoms in the primitive unit cell.
It is enough to know only the position of the atoms from the assymetrical unit cell and then use the symmetry to build the whole crystal structure.

Visualization of the crystal structure:

Size:

Nx:

Ny:

Nz:

You can define the size of the supercell to be displayed in the jmol panel as integer translations along the three crys­tallo­gra­phic axis.
Please note that the structure is represented using the pri­mi­tive cell, and not the conventional one.

Powder Raman

Powder Raman spectrum

The intensity of the Raman peaks is computed within the density-functional perturbation theory. The intensity depends on the temperature (for now fixed at 300K), frequency of the input laser (for now fixed at 21834 cm-1, frequency of the phonon mode and the Raman tensor. The Raman tensor represents the derivative of the dielectric tensor during the atomic displacement that corresponds to the phonon vibration. The Raman tensor is related to the polarizability of a specific phonon mode.

Choose the polarization of the lasers.

I ∥

I ⊥

I Total

Horizontal:

Xmin:

Xmax:

Vertical:

Ymin:

Ymax:

Data about the phonon modes

Frequency of the transverse (TO) and longitudinal (LO) phonon modes in the zone-center. The longitudinal modes are computed along the three cartesian directions. You can visualize the atomic displacement pattern corresponding to each phonon by clicking on the appropriate cell in the table below.

1

0

0

0

0

2

0

0

0

0

3

0

0

0

0

4

123

123

123

123

5

123

123

123

123

6

123

123

123

123

7

126

126

126

126

8

126

126

126

126

9

126

126

126

126

10

157

157

157

157

11

157

157

157

157

12

157

157

157

157

13

169

169

169

169

3.706e+39

15.5

6.101e+39

25.5

9.806e+39

41.0

14

169

169

169

169

3.706e+39

15.5

4.455e+39

18.6

8.161e+39

34.1

15

169

169

169

169

3.706e+39

15.5

4.730e+39

19.8

8.435e+39

35.3

16

182

182

182

182

17

182

182

182

182

18

182

182

182

182

19

192

192

192

192

20

192

192

192

192

21

194

194

194

194

22

194

194

194

194

23

194

194

194

194

24

217

217

217

217

25

217

217

217

217

26

217

218

218

218

27

231

231

231

231

28

231

231

231

231

29

231

248

248

248

30

248

248

248

248

31

248

248

248

248

32

248

261

261

261

33

261

261

261

261

9.660e+37

0.4

7.245e+37

0.3

1.691e+38

0.7

34

261

270

270

270

9.660e+37

0.4

7.245e+37

0.3

1.691e+38

0.7

35

272

272

272

272

36

272

272

272

272

37

272

283

283

283

38

283

283

283

283

4.021e+39

16.8

5.364e+39

22.4

9.385e+39

39.3

39

283

283

283

283

4.021e+39

16.8

4.678e+39

19.6

8.698e+39

36.4

40

283

327

327

327

4.021e+39

16.8

6.543e+39

27.4

1.056e+40

44.2

41

331

331

331

331

2.390e+40

100.0

0.000e+0

0.0

2.390e+40

100.0

42

352

352

352

352

43

352

352

352

352

44

352

352

352

352

45

352

355

355

355

46

356

356

356

356

47

356

356

356

356

48

356

406

406

406

No.

Char.

ω TO

ω LOx

ω LOy

ω LOz

I ∥

I ⊥

I Total

You can define the size of the supercell for the visualization of the vibration.