x ( re = 
 rare - earth metal , t = transition metal , x = b , si , al , ga , ge , sn ) 
 systems . these compounds are extremely diverse in 
 their structural and physical properties . among them
are the following : 
 ( i ) phases that form by stacking of binary cacu5-type 
 fragments and slabs of laves phases with mgzn2 and mgcu2 type ( and/or their ternary ordered derivatives ) and play an important role for improvement of 
 technological characteristics of re  ni - based negative electrode 
 material in ni 
metal hydride batteries ; ( ii ) compounds that can yield magnets appropriate for high - temperature 
 application , namely , reco7 of tbcu7 type , where 
 part of the atoms in the ca site of the cacu5 structure 
 are substituted by the dumbbells of the transition metal and the third 
 element like ti , zr , hf , cu , ga , si , and ag is required to stabilize 
 the structure and increase the magnetoanisotropy ; ( iii ) 
 magnetic materials re2co17 revealing the intergrown 
 cacu5- and zr4al3-type slabs structures 
 where the interstitial sites can be occupied by elements of iiia , 
 iva , or via groups , thus leading to the increase in curie temperature , 
 uniaxial anisotropy , and spontaneous magnetization .
the small atomic radius of boron imposes replacement of 
 the cu atom at the wyckoff position 2c in the cacu5 structure ( space group p6/mmm ; ca in 1a ( 0,0,0 ) , cu1 in 2c ( 1/3,2/3,0 ) , cu2 in 3 g ( 1/2,0 , 1/2 ) ) 
 and formation of the ordered ternary substitution derivative ceco3b2 ( ce in 1a , co in 3 g , and b in 2c ) . as relevant 
 to
the study presented herein , the ceco3b2 structural 
 unit intergrown with fragments of different structures reveals a variety 
 of borides exhibiting different degrees of structural complexity . 

for example , the family of structures where the slabs of ceco3b2 are stacked with slabs of the binary cacu5 type or laves phases are frequently encountered among ternary 
 rare - earth borides with co and ni .
formation of these structures in unexplored yet multinary 
 systems may result in unpredicted changes of expected properties , 
 and their investigation is necessary to understand and control the 
 behavior of alloys .
the diversity of cacu5-derivative 
 structures is enhanced 
 if the ternary rare - earth boride phases with noble metals are considered . 

for example , ( i ) the series of compounds formed by stacking blocks 
 of ceco3b2 and carh2b2 ( thcr2si2 ) type were observed in eu  rh 
b , y  os  b , and la  ru  b 
 systems ; ( ii ) prrh4.8b2 revealed ceco3b2-type slabs and hexagon - mesh 
 rhodium nets . in this respect ,
systems 
 containing pt and re metals were not investigated ; the only information 
 available on cacu5-type derivatives concerns the crystal 
 structure and physical properties studies for the rept4b series ( ceco4b type , p6/mmm , re = la , ce , pr , sm ) . in this article
b 
 system focused on the pt - rich concentration range where we observed 
 a series of new cacu5-related structures .
our interest 
 in this investigation was driven not only by the structural flexibility 
 and diversity of cacu5-type derivative phases but also 
 by the interesting physical properties which europium compounds may 
 exhibit , such as , for example , mixed - valence states or magnetic ordering 
 at comparatively high ordering temperatures , observed in eu - based 
 binary laves phases .
results presented 
 herein expand knowledge on a family of cacu5 derivative 
 structures to ( i ) the eu5pt18b6x phase showing a new structural arrangement formed 
 by stacking of inverse thcr2si2-type slabs with 
 cacu5- and ceco3b2-type fragments 
 along the c - axis direction , ( ii ) a new member of 
 rather simple structural series exhibiting the combination of cacu5- and ceco3b2-type slabs , namely , eupt4b where a mixed - valence state of eu has been observed , and 
 ( iii ) a new compound eu3pt7b2 , composed 
 of ceco3b2- and laves phase- ( mgcu2- ) type fragments showing interesting transport properties accompanied 
 by a relatively large sommerfeld coefficient .
0.52 g , were prepared by argon arc - melting elemental pieces 
 of europium ( 99.99% , metal rare earth ltd . ,
china ) , platinum foil 
 ( 99.9% , gussa , a ) , and crystalline boron ( 98% , alfa aesar , 
 d ) . due to the low boiling point and high vaporization of europium , 

eu weight losses were compensated by adding carefully assigned extra 
 amounts of eu before melting . for homogeneity , samples were remelted 
 several times .
part of each alloy was wrapped in mo foil , sealed in 
 an evacuated silica tube , and heat treated for 10 days at 1020 k prior 
 to quench by submerging the capsules in cold water .
single crystals suitable 
 for x - ray diffraction studies were isolated from a fragmented annealed 
 alloy of eu3pt7b2 ; however , for eupt4b and eu5pt18b6x crystals of good quality were obtained from as cast 
 samples .
crystal quality , unit cell dimensions , and laue symmetry 
 of the specimens were inspected on an axs - gadds texture goniometer 
 prior to x - ray intensity data collections at room temperature on a 
 four - circle nonius kappa diffractometer equipped with a ccd area detector 
 employing graphite - monochromated mo k radiation (  = 
 0.071069 nm ) .
no absorption 
 corrections were performed because of the rather regular crystal shapes 
 and small dimensions of the investigated specimens .
space groups were 
 determined from analysis of systematic absences performed with the 
 help of the absen program .
structures 
 were solved and refined with the aid of the wingx-1.70.00 software 
 package applying shelxs-97 and shelxl-97 programs . 
 reduced cell calculations and noncrystallographic symmetry tests were 
 performed applying the program platon2003 in order to check for higher lattice symmetry .
data collection and 
 refinement parameters for the three structures are listed in tables 1 and 2 .
isotropic ( uiso ) and anisotropic atomic displacement parameters ( uij ) are given in [ 10 nm ] .
isotropic ( uiso ) and anisotropic atomic displacement parameters ( uij ) are given in [ 10 nm ] .
x - ray 
 powder diffraction patterns collected from the eupt4b and 
 eu3pt7b2 alloys annealed at 
 1020 k for 10 days employing a guinier  huber image plate system 
 with monochromatic cu k1 radiation ( 8 < 
 2
< 100 ) revealed single - phase materials ; however , 
 for eu5pt18b6x several samples synthesized with identical conditions but 
 varying boron concentration were multiphase and contained mixtures 
 with eupt4b and/or unknown neighboring phases , suggesting 
 a different temperature range of existence ( figure 1 , supporting information ) .
structural parameters 
 obtained from rietveld powder data refinement of all three compounds validate those from the single crystal .
the third phase , eu5pt18b6x , is an interesting 
 material with respect to magnetism according to structural and bonding 
 features described below , and currently , special efforts are aimed 
 at optimization of synthesis conditions for this new compound .
a superconducting 
 quantum interference device ( squid ) served for determination of the 
 magnetization from 2 to 300 k and in fields up to 7 t where bar - shaped 
 polycrystalline specimens of about 20 mg were used .
specific heat 
 measurements on samples of about 1 g were performed at temperatures 
 ranging from 1.5 to 120 k by means of a quasi - adiabatic step heating 
 technique . electrical resistivity and magnetoresistivity of bar - shaped 
 samples ( about 1  1  5 mm )
were measured using 
 a four - probe ac - bridge method in the temperature range from 0.4 k 
 to room temperature and fields up to 12 t.
systematic extinctions in 
 the single - crystal x - ray data were consistent with three possible 
 space group types : fmmm , fmm2 , and f222 .
the structure was solved in the centrosymmetric space 
 group , and direct methods provided 9 atom positions , 3 of which in 
 further refinement were assigned to europium .
refinement of the structure 
 with all sites fully occupied resulted in rather large anisotropic 
 displacement parameters for two pt sites ( pt1 in 16 m and pt5 in 8i ) with high residual electron densities 
 ( 29  000 and 20  000 e / nm ) near these atoms . 

accordingly , the platinum atom sites were split in two near - neighboring 
 positions ( pt1pt11 0.037 nm , pt5pt55 0.029 nm ) with 
 occupancy parameters refined to 0.8/0.2 for pt1/pt11 and pt5/pt55 . 
 in the next step ,
all atoms were refined with anisotropic displacement 
 parameters except for the atoms on split sites .
subsequent difference 
 fourier synthesis revealed two significant residual peaks ( about 11  400 
 and 7000 e /  ) at a distance of about 0.2 nm from 
 the nearest platinum atoms , which were attributed to boron atoms ( 16 m and 8f ) .
refinement with both boron positions 
 fully occupied led to a low reliability factor ( rf = 0.0353 ) ; however , it showed a large isotropic thermal 
 parameter for b2 in 8f .
refinement of the b2 occupancy 
 parameter did not improve the value of uiso but reduced the occupancy factor to about 0.5 . at this point
collected 
 data were also processed in space group f222 , which 
 provides the possibility to split the b 8f site into 
 two crystallographically distinct sites 4c ( 1/4,1/4,1/4 ) and 4d ( 1/4,1/4,3/4 ) . despite almost identical agreement factors
, refinement did not 
 improve the isotropic displacement parameters of boron atoms which 
 were found to partially occupy both 4-fold sites ( occupancy ratio 
 33%:67% ) , suggesting a disordered distribution of boron atoms .
the 
 small difference between these solutions concerned also the positions 
 of b1 and the pt1pt11 split sites which all were slightly 
 shifted along the a axis due to the free x parameter of the 16k ( x , y , z ) site ( pt1 x = 0.5010(13 ) , y = 
 0.1745(2 ) , z = 0.28417(5)/pt11 x = 0.008(5 ) , y = 0.1455(9 ) , z = 
 0.2098(2 ) ; b1 x = 0.011(12 ) , y = 
 0.167(2 ) , z = 0.1071(6 ) ) . except these slender deviations , 
 refinement in f222 revealed eu and pt atoms in the 
 wyckoff positions corresponding to those in space group fmmm .
accordingly , the test for higher symmetry applying platon indicated space group fmmm . reconsidering the refinement in fmmm ,
it was found 
 out that the isotropic thermal parameter of b2 can be improved by 
 fixing the occupancy to a predetermined value : in agreement with the 
 results obtained for space group f222 , the most satisfactory 
 value for the thermal displacement parameter was observed for an occupation 
 factor fixed at 0.5 for b2 in 8f . final refinement 
 in fmmm resulted in a reliability factor as low as 
 0.0331 and residual electron densities smaller than 3710 e / nm .
the structure of eu5pt18b6x , shown in figure 1a , can be considered as built up of two kinds of blocks alternating 
 along the c direction .
one block is composed of two 
 layers of edge - connected triangular prisms which share their faces 
 to form hexagonal channels linked via the 3 net of platinum 
 and europium atoms ( figure 1b ) , while the other 
 one consists of chains of edge - connected platinum tetrahedra interlinked 
 via europium atoms ( figure 1c ) .
the 3636 kagom 
 nets of platinum atoms are slightly puckered , while 3 nets 
 are flat .
one europium atom , namely , eu3 , is located in the center 
 of a hexagon formed by pt6 ( at z = 0 ) , while eu2 
 is slightly shifted along z from the b1 atoms plane 
 ( eu2 in 8i , z = 0.39475 ; b1 in 16 m , z = 0.1070 ) .
depending on which split 
 position pt1/pt11 is locally occupied , the chains of tetrahedra are 
 bridged via ( i ) europium ( for pt1 ) or ( ii ) europium and platinum atoms 
 ( for pt11 , pt11pt11 0.2740 nm ) to form sheets extending perpendicular 
 to c. platinum tetrahedra are centered by borons ; 
 however considering that the b2 site within this block is fractionally 
 ( and randomly ) occupied ( occ .
( a ) crystal structure of eu5pt18b6x with anisotropic displacement parameters for atoms 
 from single - crystal refinement .
( b ) boron - centered pt triangular prisms ( pt2 , pt3 , pt4 , pt5/pt55 ) 
 and 3 net of pt6 accommodating eu3 atoms at z within 0.330.66 ( perspective view along the c axis ) . eu 
( c ) perspective view of boron - centered pt tetrahedra ( b2 
 in 8f ( 1/4 , 1/4 , 1/4 ) ) along the c axis . eu1pt1 and
eu1 is coordinated by 19 atoms revealing the combination 
 of an 
 elongated rhombic dodecahedron with the coordination polyhedron , which 
 is typical for the rare - earth atom in the cacu5 structure . 

both fragments are stacked via the hexagon formed by pt1 atoms ( figure 2a , table 1 ) .
the hexagonal 
 face formed by platinum atoms ( pt3 and pt5 ) is capped by a eu2 atom . 

the shapes of coordination polyhedra of eu2 and eu3 are similar : both 
 atoms are coordinated by 12 pt atoms forming two hexagonal faces of 
 the coordination sphere . while in the eu2 polyhedron they are linked 
 by pt2pt3 and pt4pt5 contacts and form a hexagonal 
 prism with the hexagonal faces capped by eu atoms ( eu1 and eu3 ) , in 
 the case of eu3 the distances between the hexagonal faces are long 
 ( > 0.4 nm ) and preclude formation of pt2pt2 and pt4pt4 
 bonds .
the coordination sphere of eu3 includes also six pt6 atoms 
 located around the waist of an imaginary hexagonal prism ; thus , those 
 18 platinum atoms form a cage elongated in the direction of the c axis ( pseudo - frank  kasper polyhedron , which can 
 also be described as two face - connected hexagonal antiprisms ) with 
 the hexagonal faces capped by eu2 atoms . in the case of eu2 ,
6 boron 
 atoms centering the rectangular faces of the hexagonal prism are located 
 too far from the central atom to infer strong bonding ( eu2b2 
 distances are 0.316 and 0.322 nm ) .
( c ) coordination 
 polyhedron of pt3 as representative of the atom environment for pt2 , 
 pt4 , and pt5 .
( d  g ) coordination polyhedra of pt6 ( d ) , b2 ( e ) , 
 pt11 ( f ) , and pt55 ( g ) .
pt1 has 10 atoms at coordination distances , and its polyhedron 
 is derived from a tetragonal antiprism ( figure 2b ) .
pt2 , pt3 , pt4 , and pt5 ( figure 2c ) are 
 surrounded by distorted icosahedra with one additional platinum atom ; 
 for all four atoms the icosahedra are formed by three almost interperpendicular 
 rectangles made of ( i ) 4 pt , ( ii ) 2 pt and 2 b , and ( iii ) 4 europium 
 atoms . the coordination sphere of pt6
resembles the coordination polyhedron 
 of cu ( 2c ( 1/3 , 2/3 , 0 ) ) in cacu5 and includes only platinum 
 and europium atoms ( in total 13 , 2 of them are at a rather long distance 
 pt62pt6 , 0.3256 nm ) ( figure 2d ) . b1 

is coordinated by 6 pt atoms forming a trigonal prism ; three eu atoms 
 are located against the triangular faces of the trigonal prism ( b1eu2 
 0.316 nm and b12eu2 0.322 nm ) ( figures 1b and 2a ) .
b2 centers the tetrahedron made 
 of near - neighboring platinum atoms ; four eu atoms complete the coordination 
 sphere , forming a tetragonal antiprism ( figure 2e ) .
the short distances between split positions ( pt1pt11 
 0.03682 nm and pt5pt55 0.03021 nm ) and their occupancies ( 0.80/0.20 
 for both pt1/pt11 and pt5/pt55 ) allow us to assume that on average 
 the pt1 and pt5 atoms are present in four of five unit cells while 
 one is filled with pt11 and pt55 .
coordination spheres for the atoms 
 when the split sites pt11 and pt55 are considered replicate the shape 
 of those with ( or for ) pt1 and pt5 differentiating slightly in coordinating 
 distances due to small shifts of atom positions , with the exception 
 of pt11 which has one more pt11 in contact distance ( figure 2f and 2 g ) .
unit cell dimensions of the eupt4b single crystal 
 and x - ray powder diffraction spectra recorded 
 from both the annealed and the as - cast alloys suggested isotypism 
 with ceco4b - type structure .
accordingly , the positions of eu and pt atoms obtained 
 from structure solution in the space group p6/mmm from the x - ray single - crystal data were consistent with 
 the ceco4b type .
refinement of the structure with 
 anisotropic displacement parameters converged to r = 0.0259 with residual electron densities smaller than 4940 e / nm and revealed full occupancy of all 
 atom sites ( table 2 ) .
the structure of eupt4b consists of ceco3b2- and cacu5-type slabs alternating along the c axis . 
 in the coordination sphere of boron ( ceco3b2-type fragment ) , six pt2 atoms are located at equal distance from 
 the central atom , forming a regular triangular prism [ bpt26 ] ( figure 3a ) .
three europium atoms are located 
 against the rectangular faces of the trigonal prism , revealing a b 
eu1 
 distance 0.32428(1 ) nm which is larger that the sum of atomic radii 
 of the elements .
thus , eu1 is surrounded by 12 
 pt2 arranged in the shape of a hexagonal prism ; two eu2 atoms cap 
 the hexagonal prisms of eu1 at a distance of 0.3720 nm from the central 
 atom ( figure 3b ) .
similar to the prototype 
 structure and to eu5pt18b6x , eu2 is located inside a rather large cage elongated 
 along z ( deu2pt2 = 0.3611 nm ) and built of 18 platinum atoms , forming two face - connected 
 hexagonal antiprisms ; two eu1 atoms cap the hexagonal faces .
both atoms in the cacu5 block ( eu2 and pt1 ) reveal 
 unusual thermal displacements : eu2 exhibits a rather large thermal 
 motion along the hexagonal axis with u33:u11(u22 ) of ca .
4:1 , while 
 pt1 shows enlarged values u11 and u22 ( u11(u22):u33 of about 
 13 ) .
no fractional or partial occupancies for atom 
 positions were observed from single - crystal data , and refinement 
 of atoms on split positions was not successful .
trial refinements 
 in the space group types with lower symmetry ( p62 m , p6m2 , p6 mm , p622 , p6/m , p6 , p6 ) yielded inferior results .
comparable features of thermal ellipsoids 
 of atoms located in the channels formed by transition - metal atoms 
 were hitherto also observed for the boride structures related to the 
 ceco3b2 type , such as in5ir9b4 ( p62 m , a = 0.5590 nm , c = 
 1.0326 nm ) and lani3b ( imma , a = 0.4970 nm , b = 0.7134 nm , c = 0.8300 nm ) ; for the latter structure , 
 the pronounced anisotropy of the atom thermal displacements prelude 
 the symmetry change upon hydrogenation . in the case of eupt4b , a possible reason may be that in order to optimize the distance 
 to its pt2 neighboring atoms eu2 is delocalized between the layers 
 of pt2 , consequently affecting ( since pt1 and eu2 are at the same 
 height ) the thermal displacements of pt1 .
enlarged displacement parameters 
 of eu1 are probably indicative of a weak rattling of the europium 
 atom within the 14-atom cage .
( a ) perspective view of the eupt4b structure along the c - axis direction emphasizing 
 the boron - centered pt2 triangular 
 prisms .
( b  d ) coordination 
 spheres of eu1 and eu2 ( b ) , pt2 ( c ) , and pt1 ( d ) .
the pt1pt2 
 bonds are omitted in b. for eu3pt7b2 , systematic extinctions characteristic 
 for the trigonal space group r3m and unit cell dimensions proposed isotypism with the ca3al7cu2-type structure .
structure solution by direct methods confirmed the arrangement of 
 the heavy atoms analogous to that observed for ca3ni7b2 ( table 2 ) .
the structure is composed of ceco3b2- and mgcu2-type fragments alternating 
 along z. each successive block is shifted with respect 
 to the former one in the ( 110 ) plane by a half unit cell .
the perspective 
 view of the eu3pt7b2 unit cell along 
 the c axis is presented in figure 4a , showing triangular prisms formed by 6 pt1 around b. the 
 ceco3b2-type slabs are formed by 12 pt1 and 
 2 eu1 atoms surrounding eu2 ( eu2pt1 0.31525 nm , eu2eu1 
 0.32982 nm ) to form the bicapped hexagonal prism ( figure 4c ) .
six boron atoms are located in front of the 
 rectangular faces of the hexagonal prism at the distance 0.3203 nm , 
 which is too long to assume bonding interaction ( compare , for example , 
 with ca3ni7b2 , where dca 
the coordination polyhedron 
 of pt2 exhibits a shape analogous to those of transition atoms in 
 the binary mgcu2-type laves phase : an icosahedron formed 
 by 6pt1 and 6eu1 reveals interatomic distances ( table 2 ) which are comparable with distances in eupt2 ( mgcu2 type , pt6pt 0.2727 nm , pt6eu 
 0.3198 nm ) .
kasper polyhedron [ pt12eu4 ] 
 of eu1 is slightly distorted in comparison with that of eu in eupt2 
 ( figure 4b ) .
( a ) perspective view of the eu3pt7b2 structure along the c - axis direction emphasizing 
 the boron - centered pt1 triangular prisms .
( b  e ) coordination 
 spheres of eu1 ( b ) , eu2 ( c ) , pt2 ( d ) , and pt1 ( e ) .
( f ) twenty - membered cage capturing ca2 
 in the ca3al7cu2 structure . as compared to the prototype ca3al7cu2 structure ,
significant changes 
 in the coordination sphere of atoms are observed for the ceco3b2-type block : ( i ) the distance between the 3636 
 kagom net formed by al atoms is long ( about 0.400 nm ) in contrast 
 to the corresponding distance between platinum atoms in eu3pt7b2 ( dpt1pt1 = 0.29959 nm ) , thus delivering a different shape of the coordination 
 polyhedron of ca ; ( ii ) due to the larger atomic radius of copper with 
 respect to b , the distance ca 
cu of 0.324 nm is sufficient 
 to indicate bonding interaction , thus increasing the coordination 
 number of ca to cn = 20 ( figure 4f ) as compared 
 to cn eu2 = 14 in eu3pt7b2 .
in contrast 
 to eupt4b , in the present structure none of the atoms show 
 significant anisotropy in their thermal vibration ( table 2 ) .
the investigated 
 structures represent three families of structures 
 revealing the cacu5-type block 
 in conjunction with other structural fragments . in the discussion 
 below
, the structures are arranged in order according to increasing 
 complexity of structural arrangements .
the eupt4b 
 structure ( ceco4b type ) ( figure 5f ) consists of alternating 
 slabs of cacu5 type ( a ) ( figure 5a ) and slabs of its ternary derivative ceco3b2 type ( b ) ( figure 5b ) . 

it is a simplest representative of the structural series described 
 in typix under the general formula rm+nt5m+3nm2n , with m = 1 , n = 1 ( m and n correspond to number of cacu5-type and ceco3b2-type blocks , respectively ) .
similar to the prototype 
 structure , the hexagonal channels filled with eu are formed by edge - connected 
 trigonal prisms [ bpt6 ] in eupt4b and alternate 
 with 18-membered platinum cages , capturing europium atoms along the c axis . while in the ceco4b structure
the b atoms 
 are located at 0.2889 nm from the central ce atom and assume bonding , 
 this distance is rather long in the europium isotype with platinum .
( a ) cacu5- and ( b ) ceco3b2-type 
 structures ( for both structures the origin is shifted by 0,0,1/2 ) . 

( c ) eupt2 structure ( mgcu2 type , space group fd3m , origin shift 1/4 , 0 , 3/4 ) . 

( e  g ) structural relationships for ( e ) 
 eu3pt7b2 , ( f ) eupt4b , 
 and ( g ) eu5pt18b6x structures .
relative arrangements of structural blocks are 
 indicated with symbols a , b , c , and d. in contrast to the relatively small unit cell and simple 
 structure 
 of eupt4b , the two remaining structures , eu3pt7b2 and eu5pt18b6x , exhibit rather large unit cells 
 and nontrivial stoichiometries .
this binary 
 compound belongs to the rhombohedral branch of a structural series 
 formed within composition range rm2rm5 by stacking the fragments of cacu5 type and laves phase 
 and can be described by the formula r2m+nm4m+5n , where m accounts for the number of laves - type 
 slabs ( r2m4 ) and n is a number 
 of cacu5-type slabs . since only the structures with m
= 1 have been observed for binary structures , the ternary 
 representatives of this series which are built by combining ceco3b2- and laves - type structural slabs ( ca3ni7b2 , ce2ir5b2 ) 
 follow the formula r2+nm4 + 3nx2n : 2mgcu2 + nceco3b2 .
cu 
 system . in ca3al7cu2-type structures with boron ( ca3ni7b2 , eu3pt7b2 ) , the transition - metal 
 atoms adopt the sites of aluminum in the mgcu2-type block 
 and boron atoms replace copper in the cacu5-type block . 

figure 5e shows the arrangement of ceco3b2-type slabs ( b ) and mgcu2-type slabs 
 ( c ) in the eu3pt7b2 structure .
it can 
 be considered as an intergrowth of three kinds of structure blocks 
 ( figure 5 g ) : one is a cacu5 type 
 ( a ) , the second having the atom arrangement of its ordered ternary 
 derivative ceco3b2 ( b ) , and the third reveals 
 the slab of the site - exchange variant of the thcr2si2 structure with b atoms adopting the sites of cr ( d ) .
the 
 body - centered structure of thcr2si2 ( space group i4/mmm , th in 2a : 0 , 0 , 
 0 ; cr in 4d : 0 , 1/2 , 1/4 , si in 4e : 0 , 0 , z ) , an ordered variant of baal4 , is widely distributed among ret2x2 compounds 
 ( t = transition metal , x = p element ) . the unit cell of thcr2si2 can be described as a stacking of infinite layers of interconnected 
 tetragonal [ cr4 ] pyramids around the si atom ( wyckoff position 
 4e , commonly named as pyramidal site ) running perpendicular 
 to the c axis with a layer of thorium atoms between 
 these pyramids .
cr atoms are tetrahedrally surrounded by four si atoms 
 ( wyckoff position 4d , tetrahedral site ) .
chemical 
 bonding , atomic site preferences as a function of electronegativity 
 of the constituent elements , size effect with respect to the structural 
 stability , phase widths , and physical properties have been studied 
 for different combinations of the elements and particularly developed recently for pnictides due to discovery 
 of superconductivity in the series of iron arsenides with thcr2si2 type .
ternary rare - earth borides with 
 a thcr2si2-type structure are quite rare except 
 for few representatives , such as reco2b2 ( re 
 = y , la , pr , nd , sm , gd  er ) , refe2b2 ( re = y , gd 
these structures the transition metal occupies the atom site of cr 
 and boron is placed in the position of si .
accordingly , each t atom 
 is coordinated by four b atoms ; a recent reinvestigation of the 
 thcr2si2 type laco2b2 ( a = 0.36108 nm , c = 1.02052 nm , z = 0.3324(5 ) ) revealed a relatively short co 
b bond 
 ( e.g. , 0.20 nm ) , indicating certain compression of the cob layer along 
 the c axis and an elongated la 
there are two baal4-type derivative structures encountered 
 for composition ret2x2 : the structures of thcr2si2 ( described above ) and cabe2ge2 , which is built by intergrowth of thcr2si2-type slabs and slabs of its site - exchange variant along the 
  direction ( and thereby one - half of t and x atoms occupy the 
 tetrahedral and pyramidal sites and vice versa , respectively ) . as 
 reported by parthe et al .
, formation 
 of compounds revealing only inverse thcr2si2-type arrangement is rare ; however , this arrangement occurs as slabs 
 in intergrown structures ( for example , in cenisi2 , bacusn2 , la3co2sn7 ) . among borides , 

formation of inverse thcr2b2-type structure 
 was found for znir2b2 where layers of edge - connected 
 [ bir4 ] tetrahedra ( ir in 4e ( 0 , 0 , z , z = 0.37347 ) , b in 4d ( 0 , 1/2 , 1/4 ) ) are separated by 4 networks of zinc atoms 
 ( zn in 2a ( 0 , 0 , 0 ) ) .
similarly , in eu5pt18b6x , four atoms of platinum form tetrahedra around boron 
 atoms , however exhibiting a higher degree of compression along the b axis : the pt1b distance in the eu5pt18b6x is 0.1978(1 ) nm ( and 
 0.2239(8 ) nm for pt11 in split position ) as compared to the length 
 of the ir 
b bond of 0.2150 nm in znir2b2 showing the tetrahedral angles 2  137.07 , 2  105.25 , 
 2  90.27 in eu5pt18b6x ( 2  126.09 , 2  100.79 , 2 
  102.91 for pt11 ) and 4  121.68 , 2  
 87.11 in znir2b2 .
because of limited spatial 
 dimension of the thcr2si2 slab in the eu5pt18b6x structure 
 ( i.e. , one - half a unit cell cut along ( 111 ) ) , the tetrahedra do not 
 form infinite layers but are arranged in one - dimensionally linked 
 chains running infinitely along the a axis .
previous 
 theoretical studies based on mlliken overlap population analysis 
 suggested that the element with greater electronegativity is more 
 strongly bound in the 4e site .
the atoms site preferences in the structures of three discussed 
 borides ( i.e. , laco2b2 , znir2b2 , and thcr2si2 block in the eu5pt18b6x ) are consistent 
 with the electronegativity on the pauling scale of boron , relative 
 to those of transition elements co , ir , and pt . among a large 
 family of reported ternary rare - earth transition - metal 
 borides
, the eu phases are usually missing . for only a few cacu5-type derivative ternary borides with europium
have the precise 
 structural parameters and physical properties been investigated . 
 these are ( i ) the eu2rh5b4 and eu3rh8b6 structures composed of the ceco3b2- and carh2b2-type fragments ,
( ii ) eu3ni7b2 of ceco4b type exhibiting one nickel site partially occupied 
 by a mixture of europium and nickel atoms . in both eu2rh5b4 and eu3rh8b6 compounds ,
the eu atoms have been found 
 in the divalent state , revealing magnetic moments only slightly smaller 
 than the theoretical value of 7.94 b .
euir2b2 of carh2b2 type can be considered as a metal - deficient derivative 
 structure of ceco3b2 ; according to a plot of 
 the unit cell volumes vs lanthanide atomic number , eu was found to 
 be divalent . comparing the interatomic 
 distances and relating them to the magnetic properties
, one can see 
 that in both eu2rh5b4 and eu3rh8b6 ( eu 4f ) the eu 
eu 
 distances are 0.3207 , 0.3654 nm and 0.3113 , 0.3614 nm respectively , 
 while eu  rh distances vary between 0.3079 and 0.3256 and 0.3054 
 and 0.3279 nm .
euir2b2 shows similar distance 
 variations : the eu  eu bond is 0.3852 nm , and the distances 
 eu  ir are 0.3058 and 0.3313 nm .
the differences in bond length 
 analogous to eu2rh5b4 , eu3rh8b6 , and euir2b2 can 
 be found in eupt4b , where one eu atom , namely , eu1 ( ceco3b2-type block ) exhibits rather short contacts with 
 pt2 of 0.31605 nm , whereas eu2 ( cacu5-type block ) exhibits 
 pt neighbors at 0.32428 and 0.36113 nm .
there is one rather long bond 
 between europium atoms eu1eu2 0.3720 nm in eupt4b .
the deviations of the lattice parameters of the europium compound 
 from the lanthanoid contraction in the rept4b series also 
 confirm the assumption that eu is not trivalent in eupt4b ( figure 6 ) .
similarly , in eu5pt18b6x the eu  eu 
 contacts are long and the distances between eu and pt atoms are rather 
 heterogeneous , ranging within 0.3128 and 0.3611 nm .
for the europium compound , lattice parameters were obtained from 
 rietveld refinement of single - phase eupt4b alloy used 
 for physical properties measurements .
in order to obtain the electronic configuration 
 ( ec ) of the eu 
 ions and thus the magnetic state , the magnetization of eupt4b and eu3pt7b2 was measured and 
 analyzed in detail .
the temperature dependence of the magnetization 
 and susceptibility for various fields is presented in figures 7 and 8 , respectively .
magnetic 
 susceptibility data  = m / h scale pretty well for 0.1 , 1 , and 3 t above the ordering temperature 
 being indicative that the samples are free from magnetic impurities 
 and other magnetic secondary phases ; data are displayed in figure 7 as  vs t for the 3 t run , only .
temperature - dependent magnetization of eupt4b ( a ) and 
 eu3pt7b2 ( b ) . both eupt4b and eu3pt7b2
eu3pt7b2 shows a transition at 57 k , while eupt4b 
 orders below 36 k as determined from low - field magnetization measurements 
 and corresponding arrott plots ( see below ) .
the magnetic susceptibility 
 in the paramagnetic region is accounted for by the modified curie 
weiss 
 law  = 0 + c/(t  p ) . the paramagnetic curie temperatures 
 p = 60 and 40 k together with effective magnetic 
 moments eff = 8.0 and 7.1 b per 
 eu atom ( derived from the curie constant , c ) , were 
 obtained as a result of least - squares fits to the susceptibility data 
 of eu3pt7b2 and eupt4b , 
 respectively ( solid lines , figure 7 ) , with 
 a temperature - independent susceptibility 0 of about 
 3  10 cm / g for both compounds . 
 while the paramagnetic moment of eu3pt7b2 recounted per one europium atom is almost that expected for 
 the theoretical value of the free eu ion ( eff = 7.94 b ) the effective moment obtained 
 for eupt4b is significantly smaller , pointing to a mixed- 
 or -intermediate valence state of eu . the fact that eu in eupt4b possesses two inequivalent lattice sites suggests that the 
 more static case ( i.e. , mixed valence ) might account .
in such a case , 
 the system can be treated as a mixture of eu and eu atoms located either at the 1(a ) or the 1(b ) site . following 
 this approach , the paramagnetic moment of eu in eupt4b 

we use a mean literature value for the effective moment of eu with eu = 3.5 b rather than the theoretically vanishing 
 effective magnetic moment of a pure eu state .
the fraction x of eu atoms in the 2 + state is thus estimated to be x = 0.75 , whereas a vanishing effective moment for the eu state gives a fraction of 80% in the eu state . 

although the latter is presumably an overestimate , analysis of the 
 saturation magnetization and heat capacity ( see below ) supports these 
 assumptions .
the temperature - dependent magnetization , displayed 
 in figure 8 for various fields , exhibits for 
 eupt4b significant irreversibilities of the magnetization 
 for zero - field 
 cooling ( zfc ) and field cooling ( fc ) which disappear only for fields 
 larger than 1 t. these irreversibilities can be attributed to domain 
 wall pinning associated with a remarkable hysteresis and a coercivity 
 of 0.32 t at 2.8 k ( see figure 9 ) as a consequence 
 of the magnetocrystalline anisotropy due to the cacu5 building 
 blocks . on the contrary , there is hardly any difference for zfc and 
 field cooling detectable even in the low - field regime for eu3pt7b2 yielding also reversible magnetic isotherms 
 without a significant hysteresis , which means the coercivity is 3 
 orders of magnitude smaller than for eupt4b .
a comparison of the magnetic isotherms of both compounds 
 is shown 
 in figure 10 , where the isotherms are displayed 
 as m versus h / m plots ( arrott plots ) .
it should be noted that in the case 
 of eupt4b with the pronounced hysteresis only the demagnetization 
 data , where the rotation of the magnetization plays the dominant role , 
 are used for the arrott plots .
deviations from the expected linear 
 behavior of the arrott plots are observed frequently , see , e.g. , ref ( 59 ) and references therein , 
 in particular with a curvature symmetrical with respect to tc , i.e. , a negative curvature at t < tc and a positive curvature at t > tc .
this is indeed observed 
 for eupt4b ( figure 10a ) , and such 
 symmetrical deviations about tc can be 
 attributed to spatial variations of the magnetization and/or spin 
 fluctuations . as we have a stoichiometric compound these deviations 
 may arise from fluctuating moments associated with significant thermal 
 motions of eu2 along the hexagonal axis and/or with the proposed weak 
 rattling of the eu1 atoms within the 14-atoms cage rather than from 
 a heterogeneous magnetization .
on the other hand , it was shown that 
 a substantial uniaxial magnetocrystalline anisotropy causes a remarkable 
 downturn of the arrott plots below tc . in this model
, the negative curvature of the 
 arrott plots below tc can be accounted 
 for as a result of domain rotations in the nonoriented crystallites 
 and against the random but uniaxial anisotropy fields ha .
a rather simple way to estimate ha in terms of this model is to extrapolate the linear part 
 of the arrott plot backward to obtain the spontaneous magnetization , 
 and taking 91.3% of that value gives with a parallel forward extrapolation 
 an intersection with the experimental arrott plot , yielding a value 
 for h / m where this particular field 
 corresponds to the anisotropy field ha ( see the dashed lines in figure 10b ; for 
 further details see ref ( 59 ) ) . using this simple estimate we obtain for eu3pt7b2 an anisotropy field of about 1
although the magnetization at 7 t at 2.8 k is not in a fully 
 saturated 
 state for both compounds we use that value for the saturation magnetization ms , while the spontaneous magnetization m0 is obtained from the backward extrapolation 
 of the arrott plot as mentioned above .
this yields for eu3pt7b2ms = 7.43 
 b / mol - eu and m0 = 7.24 
 b / mol - eu and for eupt4b ms = 5.13 b / mol - eu and m0 = 3.85 b / mol - eu . both the saturation 
 as well as the spontaneous magnetization of eu3pt7b2 is well above the value expected for a eu ion ( ms = gj = 7 b ) , indicative that a substantial 5d conduction electron polarization 
 contributes to the total moment .
the significant smaller saturation 
 and spontaneous moment of eupt4b indicate , in line with 
 results for the effective moments , a mixed - valence state . under the 
 assumption that the conduction electron polarization is of similar 
 magnitude in both compounds ,
the ratio of their saturation moments 
 indicates that 69% of the eu ions are in the eu state 
 in eupt4b , while a somewhat smaller fraction of about 53% 
 is obtained using the ratio of the spontaneous moments .
the latter 
 is an underestimate caused presumably by the large high - field susceptibility 
 of eupt4b also visible in the larger slope of the arrott 
 plot in comparison to that of eu3pt7b2 .
further information on the ground state properties 
 of eupt4b and eu3pt7b2 can be gained 
 from specific heat measurements .
temperature - dependent specific heat cp of eupt4b ( a ) 
 and eu3pt7b2 ( b ) .
low - temperature details together with least - squares 
 fits ( solid lines ) based on a ferromagnetic spin wave model . both eupt4b and eu3pt7b2 exhibit distinct -like 
 anomalies around 35 and 57 k , respectively , 
 idealized in figure 11 by solid lines , which 
 can be attributed to a second - order phase transition of magnetic origin . 

a nonmagnetic isostructural compound to be used as an analogue was 
 not available for a full comparative analysis .
however , it is easily 
 seen that the height of the anomaly cp = 56 j / molk in the case of eu3pt7b2 corresponds well to a mean - field - like 
 transition of a j = 7/2 system ( cp = n*5r[j(j + 1)/(2j + 2j + 1 ) ] , where n is 
 the number of re atoms / mol .
considering three eu atoms 
 per mole yields cp = 60.39 j / molk , 
 in fair agreement with experiment .
the jump of the specific heat 
 of eupt4b right at the magnetic phase transition temperature 
 is only about 13 j / molk in comparison with the expected value 
 of 20.13 j / molk .
this reveals that only a fraction of the eu 
 ions is involved in magnetic ordering ( i.e. , 65% eu and 
 35% eu ) , which is in line with the broad range of estimates 
 from the magnetic data where we obtained 53% eu as a 
 lower and 80% eu as an upper limit .
low - temperature 
 heat capacity data were analyzed in terms of a 
 ferromagnetic spin wave model including a spin wave gap  , i.e. , cp = t + t + t  exp(/t ) with 
  the sommerfeld value and  proportional to the low - temperature 
 debye temperature d ( d = ( ( 1944n)/ ) . 

results of least - squares fits are shown as solid lines 
 in the insets of figure 11 , revealing excellent 
 agreement with  = 28 and 206 mj / molk , resulting 
 in an effective 68 mj / molk per europium atom ,  
 = 0.000222 and 0.000702 j / molk ,  = 0.36 
 and 0.46 j / molk , and  = 12.6 and 10 k for 
 eupt4b and eu3pt7b2 , respectively . 

both eu - based compounds are characterized by sommerfeld values well 
 beyond simple metals , pointing to strong electron correlations induced 
 by intra - atomic 4f5d exchange in eu ions in combination 
 with a significant hybridization of eu and pt 5d orbitals .
the latter 
 is also corroborated by the high saturation magnetization of eu3pt7b2 which significantly exceeds the 
 expected saturation moment of free eu ions .
we note that 
 largely enhanced sommerfeld values due to f  d exchange have 
 been reported for various gd intermetallics such as gd3co , gd3rh , and gd  ni binaries .
we studied the electrical resistivity  
 of eupt4b and eu3pt7b2 from 0.3 k to room 
 temperature ( see figure 12 ) .
data evidence 
 metallic behavior in both cases with rather low overall resistivities . 
 unlike eu3pt7b2 , in the resistivity 
 curve of eupt4b
a slight change of slope is observed at 
 around 240 k , pointing to some additional , unanticipated scattering 
 processes . for both compounds , anomalies of (t )
figure 13 represents the magnetic field 
 dependence of the resistivity of eu3pt7b2 and eupt4b .
although the anomaly in both compounds 
 is of ferromagnetic origin , application of a magnetic field causes 
 different responses of the resistivity .
the resistivity of eupt4b decreases slightly in the entire temperature range studied , 
 and the (t ) anomaly is suppressed by fields 
 of 6 t. magnetoresistance of eu3pt7b2 , on the other hand , is positive at low temperatures as well as in 
 the high - temperature paramagnetic range and negative only in a narrow 
 region around the transition temperature . despite the ferromagnetic 
 ground state of eu3pt7b2 leading , 
 in general , to a negative magnetoresistance , the classical magnetoresistance 
 can overcompensate the one originated by magnetic interactions , resulting 
 in an increase of (t ) with increasing magnetic 
 fields , at least for certain temperature ranges .
the dominance of 
 the classical magnetoresistance in eu3pt7b2 as compared to eupt4b might follow from the fact 
 that the overall resistivity , specifically the residual resistivity , 
 is much lower here than in the case of eupt4b .
temperature - dependent 
 electrical resistivity of eu3pt7b2 ( a ) and eupt4b ( b ) as a function 
 of magnetic field . to quantitatively evaluate 
 (t ) of eu3pt7b2 ,
experimental data were first 
 split into two parts , i.e. , a low - temperature region ( below the magnetic 
 phase transition ) and a high - temperature paramagnetic part . in the 
 paramagnetic regime the behavior of the resistivity of eu3pt7b2
grneisen 
 term corresponding to the resistivity originated from scattering of 
 conduction electrons on phonons and  * = 0 + mag , with mag describing the 
 interaction of conduction electrons with localized magnetic moments . 
 in the absence of crystalline electric field effects
a debye temperature 
 d = 312 k and  * = 32 cm 
 were obtained from a least - squares fit in a temperature region over 
 80 k ( dashed line , figure 12 ) . in the low - temperature 
 region ,
the resistivity of a ferromagnetic material with spin wave 
 dispersion   k is expected 
 to behave like t . moreover , as 
 a consequence of the enlarged gamma value , derived from the specific 
 heat measurements , the electron 
electron scattering should 
 also be taken into account and in turn be proportional to t at low temperatures as well , making it impossible 
 to distinguish between both phenomena . as a result of the fit using 
  = 0 + at , 
 0 = 7.64 cm was obtained .
the low - temperature part of the resistivity of eupt4b 
 is in good agreement with the temperature dependence of a ferromagnetic 
 material .
experimental data are well accounted for by = 0 + bt(1 
 + ( 2t/))e with  = 12.8 k as the width of the gap in 
 ferromagnetic spin wave spectrum .
the high - temperature part , on the 
 other hand , displays some characteristic curvature observed in systems 
 strongly influenced by the crystalline electric field , supporting 
 the mixed - valence hypothesis .
three new ternary phases 
 have been observed for the first time 
 in the eu  pt  b system from arc - melted alloys annealed 
 at 1020 k. crystal structures have been studied by x - ray single - crystal 
 diffraction and validated by rietveld refinements of x - ray powder 
 diffraction data .
the results beautifully illustrate the structural 
 diversity and versatility of a cacu5-type derivative family 
 of structures , revealing three different structural series .
eupt4b forms a ceco4b type and is composed 
 of cacu5- and ceco3b2-type fragments 
 stacked along the c axis ; eu3pt7b2 is built of ceco3b2- and mgcu2-type slabs and exhibit an ordered variant of puni3 , namely , ca3al7cu2 type with al 
 and cu atom sites occupied by pt and b atoms , respectively . in both 
 compounds
unique stacking of cacu5- , ceco3b2- , and thcr2si2-type slabs 
 has been uncovered 
 in the eu5pt18b6x structure ; the atom arrangement in the thcr2si2-type block is rare for the distribution of pt and b atoms 
 on si and cr atom sites , respectively ( baal4-type derivative 
 structures ) , thus changing the coordination of the transition metal 
 as compared to other representatives of a large family of thcr2si2-type compounds and exhibiting chains of edge - connected 
 platinum tetrahedra [ bpt4 ] running along the a - axis direction . with respect to eu 
eu linkage , the europium 
 atoms are interconnected infinitely along the c axis 
 in both eupt4b and eu3pt7b2 structures . the eu5pt18b6x structure exhibits the repeating blocks of 5 fused 
 polyhedra of eu linked in a sequence ...
thcr2si2ceco3b2cacu5ceco3b2thcr2si2 ... ; the units are shifted for a 1/2a along the b - axis direction with respect to each 
 other , forming blocks which are interlinked via thcr2si2-type fragment along the c axis .
positioning 
 of the b atom on the cr wyckoff site in the thcr2si2-type slab and consequently its nearest tetrahedral coordination 
 plays a decisive role in formation of eu5pt18b6x structure , serving as a linking 
 entity ( figure 14 ) for the repeating structural 
 units of fused eu polyhedra .
although eupt4b exhibits a feromagnetic ordering at 
 relatively high temperature of 36 k , a mixed - valence state for eu 
 is observed . in a static case of valence distribution
one would expect 
 50% eu and 50% eu as a consequence of obvious 
 difference between the coordination polyhedra for two eu atoms unlike 
 our case , where magnetic and specific heat measurements purpose more 
 complex valence behavior .
time - dependent studies are needed to distinguish 
 between a static and a more complex dynamic case of mixed - valence 
 state . in eu3pt7b2 ,
the eu 
 atoms are 
 in the 2 + state and the compound orders feromagnetically at around 
 57 k. the electron part of specific heat was found to be 206 mj / molk , pointing to the existence of strong electron correlations .
 three novel europium 
 platinum borides have been synthesized by 
 arc melting of constituent elements and subsequent annealing . 
 they 
 were characterized by x - ray powder and single - crystal diffraction : 
 eupt4b , ceco4b type , p6/mmm , a = 0.56167(2 ) nm , c = 0.74399(3 ) nm ; eu3pt7b2 , ca3al7cu2 type as an ordered variant of 
 puni3 , r3m , a = 0.55477(2 ) nm , c = 2.2896(1 ) nm ; and 
 eu5pt18b6x , a new unique structure type , fmmm , a = 0.55813(3 ) nm , b = 0.95476(5 ) nm , c = 3.51578(2 ) nm . 
 these compounds belong to the cacu5 family 
 of structures , revealing a stacking sequence of cacu5-type 
 slabs with different structural units : cacu5 and ceco3b2 type in eupt4b ; ceco3b2 and laves mgcu2 type in eu3pt7b2 ; and cacu5- , ceco3b2- , and site - exchange thcr2si2-type slabs in 
 eu5pt18b6x . 
 the striking motif in the eu5pt18b6x structure is the boron - centered pt tetrahedron [ bpt4 ] , which build chains running along the a axis and plays a decisive role in the structure arrangement by linking 
 the terminal fragments of repeating blocks of fused eu polyhedra . 
 
 physical properties of two compounds , eupt4b and eu3pt7b2 , were studied . both compounds 
 
 were found to order magnetically at 36 and 57 k , respectively . for 
 eupt4b a mixed - valence state of the eu atom 
 was confirmed 
 via magnetic and specific heat measurements . 
 moreover , the sommerfeld 
 value of the specific heat of eu3pt7b2 was found to be extraordinarily large , on the order of 0.2 j / mol  k2 . 
Introduction
Experimental
Section
Determination and Analysis of Crystal Structures
Structural Relationships
Magnetic Properties
Specific Heat
Electrical
Resistivity
Summary