Isotypic compounds: All references Y a K , 76 RCo2Ga: R = La, Pr
The PrCo2Ga structure, shown in fig. 40, can be considered as an intergrowth o f 8340:CeCo3B2-type slabs (see fig. 58) and CsCl-type slabs according to
PrCo3Ga2 + P r C o = 2 PrCo2Ga.
-0--,o-0<>0-,::,'
ù - , 0 , - , o , - 0 , - , , - ~ 0
0 ~" " " T JÖ "~-" "'""\'!" 0 ' " " CeC°3Bz
-,",--(-~4-," ,--(-'~-i-," »-("'~ . . .
_t'-~-_,--,-!~"~-,",- :('~--,--,
c~M M - - ' i M . , / " - " i M J " - "
ù ,O,±, ,-,0,":, ,-,O 0'-" '-'Ö '-/''''-'O'-'
-,:>0-<-»-0<,'-0
CsCt
CeCo3B 2
CRYSTAL STRUCTURES AND CRYSTAL CHEMISTRY 193
0
"i 0
Height R T M
~ .4,5
~ ~~~~~
~~o~4~
.,o~~~~ o~.~~ ... ~~.~~,~~~~
YPd2Si RT2M
Fe3C type derivative Re3B type derivative
ot:)16, Pnma oC16, Cmcm
a=7.300, b=6.927, c=5.499 Ä C~bypdzSi
Fig. 41. Segments of YPd2Si structure (0 < y < ½) with Fe3C-type derivative and of hypothetical RT2M structure (0 < z < ½) with Re3B-type derivative. The !atter structure with T and M sites interchanged is found with YNiA12.
prism formed of two Y and four Pd atoms (six Fe atoms). The Fe3C-type is very common in binary R3T compounds (78 examples). Structurally related to the Fe3C-type is the Re3B-type (ArBR, 60) found, for example, with Pu3Co, Zr3Co and Ca3Zn. Both structure types are characterized by centred trigonal prisms and each atom forming a prism participates in two prisms [prism linkage coefficient LC = 2, see Parthé (1981) and Parthé and Moreau (1977)]. The two structures differ only in the way the prisms are linked. In Fe3C and YPd2Si the centred prisms are connected to form nets (two prism nets perpendicular to b, only one shown in fig. 41), but in Re3B they form isolated infinite columns of prisms. Using the concept of periodic unit cell twinning the Fe3C-type can be derived from a hexagonal close packed base structure and the Re3B-type from a cubic close packed base structure (Chabot and Parthé, 1978). The close relationship of the binary structure types suggests the possible existence of a ternary Re3B-type derivative RT2M, a segment of which (0 < z < ½) is shown in the right-hand part of fig. 41 (four columns parallel to a per unit cell, only two of which are shown in fig. 41). No silicide or germanide is known with this structure*, but an aluminide exists. However, here the T sites are occupied by A1 atoms and the M sites by T atoms. See 7567:YNiAI2 and a further discussion with 7350:Sc3Ni4Ge4 .
*It should be mentioned, that a Re3B-type derivative structure has reeently been found for TaCo2B (StRN, 79; NoRS, 82), however, as compared to the drawing given on the right of fig. 41, with the a and b axes doubled and space group Cm2m.
7533 YPd2Sn cF16 a = 6.720*
or Fm3m
yis, clpd}4, 4)c]Sn[.8c]
Is J J, 82
MnCu2Al (Heusler phase)-type = CsCl-type derivative
*From unpublished single crystal studies by K. Yvon.
Isotypic compounds:
RNi2Al: R = Sc a) RPd2In: R = yb)
RPd2Sn: R = Gd b), Tb b~, Dy b~, Ho ó), Er s~, Tm b), Yb b), Lu b), yb) RPd2Pb: R =Yb)
~)GoR, 68 b)IsJJ, 82
The YPd2Sn structure with MnCu2A1 (Heusler phase)-type is shown in fig. 42. This face-centred structure is a superstructure of the CsCl-type.
r" YPd2Sn
MnCu2A[ (Heus[er) type cF16, FmSm
- - a:6Z20 Ä
-
©©o
Y
Fig. 42. The YPd2Sn structure with MnCu2A1 (Heusler phase)-type.
Eight structure types are found with composition 7567:
- ScRhSi2-type, oP16, -LuRuB2-type, oP16,
-CeNiSi2 (or BaCuSn2)-type, oC16, -TbFeSi2-type, oC16,
-NdNiGa2-type, oC16,
-MgCuA12-type, oC16 (see YNiA12), -TiMnSi2-type, oP48 (see ScMnSi2), ZrFeSi2-type, oC96 (see ScFeSi2).
CRYSTAL STRUCTURES AND CRYSTAL CHEMISTRY 195
7567 SeRhSi2 oP16 a = 6.292 ChBYP, 81
or Pnma b = 4.025
ScRh[,4tl ~ Si[2+ 21 c = 9.517
Related to NbCoB2-type (Ku, 76), YZn3-type (MIR, 68) derivative*
*The NbNiP2-type (GhGPS, 81) has the same Pearson symbol, space group and Wyckoff positions, but is neither isotypic to ScRhSi2 nor to NbCoB>
N o other isotypic RTM2 compounds are known
The structure o f ScRhSi2, a YZn3-type derivative, shown on the upper left of fig.
43, is composed of (-R4T2M2 and r-R4T2M2 columns which are presented in fig. 14 and discussed with 6067:Sc2CoSi» These characteristic columns are identical with those found in 6750: ScRhSi with TiNiSi-type, where the Rh atoms are in the centres of silicon tetrahedra. In ScRhSi2 the Si atoms form infinite Si-Si zig-zag chains parallel to b.
The ScRhSi 2 structure is related to the NbCoB2 structure (Ku, 76), shown in the lower part of fig. 43, both having the same space group, the same equipoints and the same structural features. However, the much shorter B-B bonds in the zig-zag chains lead not only to different unit cell ratios, but also to considerable changes in the adjustable parameters of the atom positions. The axial ratios for the compounds of interest are as follows:
ScRhSi2 YZn3 NbCoB2
c/a
1.512 1.51 1.356c/b
2.364 2.30 2.627A comparison of the ScRhSi2 and NbCoB2 drawings in fig. 43 indicates that in the boride the columns not only have a different shape but are also rotated with respect to each other. It appears therefore appropriate to call both structures not isotypic, but only geometrically related. K u z m a (1976) has described the N b C o B 2 structure as an arrangement of intergrown segments o f CrB and FeB. In the right-hand part of the NbCoB2 drawing the trigonal prisms are emphasized.
The third structure shown in fig. 43, the 7567: LuRuB 2 structure, differs from the first two in various aspects. The axial ratios are quite different
(c/a
= 1.082,c/b
= 1.202) and the boron atoms now form dumbbells instead of zig-zag chains.One recognizes boron-centred trigonal prisms formed of three Lu and three Ru atoms. The structure can be described as being built up of slabs of the FeB structure*
which are shifted with respect to each other.
*To obtain a projection of the FeB structure similar to that of LuRuB 2 in fig. 43, the structure has to be projected approximately along (184).