Stannides structure types

Some of the plumbides within the Yb3Rh4Sni3 family show complex superstructures. They have the compositions R- Rh Pb ip (R = Y, Pr, Nd, Sm, Yb) (Venturini et al., 1986) and the structure is closely related with the Eri tSnIEr4Rhfl Snis structure (Hodeau et al., 1984 Vandenberg, 1980). This complex structure type crystallizes in space group I4i/acd. All investigated stannide crystals are twinned by recticular pseudomerohedry. So far, the corresponding plumbides have only been studied by X-ray powder diffraction. 

So far more than 180 rare earth-transition metal-plumbides have been reported. They crystallize with 23 different structure types. Apart from the few lead rich plumbides with YbsRlpSnis and related structures, only plumbides with 33 at% or even lower lead content have been reported. Some ternary systems exhibit large liquidus ranges in the lead rich regions at 870 °C. Through phase analytical investigations at lower temperatures one will certainly get access to new lead rich phases. In view of the more than 500 and 850 rare earth-transition metal-stannides and indides, respectively, the lead based systems certainly have a great potential for many more phases to be discovered. 

Only YbAgPb and La4Ni3Pb4 show peculiar structure types, which have first been observed for a plumbide. All other plumbides exhibit relatively simple structure types, which have been observed also for silicides, germanides, stannides, gallides, or indides. We expect that lead characteristic structures will form in the lead rich parts of the ternary systems, similar to the gallium and indium based phase diagrams. 

The phase equilibria of the Sc-M-Sn (where M = Ru, Rh, Pd, Os, Ir, or Pt) systems are not known. One or two ternary compounds have been synthesized in each of the systems. These data are shown in table 21. Espinosa et al. (1982) reported on the tetragonal ternary compounds of a tentative composition ScMi.jSns.e (M=Ru, Rh, Ir) giving their pseudocubic cell parameters (table 21). Probably, these compounds are isotypic with Er5 j Rh6Sni8+ found by Hodeau andMarezio (1984), however, additional investigations are necessary to determine the proper composition and structure of these stannides. Two and three different crystal structure types have been reported for ScPdSn and ScPtSn, respectively (see table 21). This might be an indication for a polymophism of these compounds. 

Interconnection of the structure types of stannides with the other types 4. Properties of the ternary stannides... 

The RsSn4 compounds crystallize in the Sm5Gc4 structure type which is a relative of the U3Si2 type and in turn of the AIB2 structure type. There are no data on the stannides with this composition for Ho, Er, Tm for the remaining rare earths such information has been found. Most likely this fact can be explained by the insufficient investigation of the systems with Ho, Er and Tm in this range. 

Only three stannides of RSn composition with known structure form in the systems R-Sn. They are LaSn, EuSn (CrB strueture type) and YbSn (CuAu structure type). The structure of the remaining RSn compounds (R=Pr, Nd, Tb) is unknown (table 1). 

The MgCotSn structure is the superstructure to the AuBes type, which is the derivative from the MgCu2 type. Only one stannide, LuNi4Sn, is known to crystallize in this structure type. 

The R(Cuo,72Sno28)i3 stannides, where R=La, Ce, Pr, Nd, crystallize with theNaZni3 structure type. 

The coordinates of atoms have not been determined for flie stannides of fliis structure type. The unit cell of La6ConGa3 can be presented as a combination of flie Ct5B3 and U(Nio.68Sio,32)ii structure type fragments which are perpendicular to the Z axis. 

The RFefiSng (R = Y, Gd-Tm, Lu) and RCoeSng compounds crystallize in YCogGeg type, while the RMnsSne (R = Sc, Y, Gd-Tm, Lu) stannides crystallize in HfFeeGeg structure type. 

The RgCusSng stannides (R=Y, Gd-Tm) (Thirion et al. 1983, Skolozdra et al. 1984a) crystallize in this structure type. From a single crystal study it was established that TmsCugSng also has a monoclinicly deformed structure which is derived finm the GdfiCugGeg type. The correspondence between the lattice parameters of monoclinic and orthorhombic phases is appreciable if the first one is considered to be in I2/m SG (see table 5). 

The structure is a superstructure of the Calna type. The YbZnSn and EuCdSn stannides crystallize in this structure type. 

The RMSn (M = Rh, Pd, Ir, Pt) compounds crystallize in this structure type. These structures are characterized by an ordered distribution of R and Sn atoms. Such a distribution of different atoms in the 3(g) and 3(f) sites was observed for the first time in ZrNiAl which is a superstructure of the FcaP type (Krypyakevich et al. 1967). The YbCdSn stannide crystallizes in this structure type as well. 

The RAuSn (R=Ho, Er, Tm, Lu) stannides belong to this structure type. 

The stannides of this structure type were studied by Skolozdra et al. (1988b), Komarovskaya et al. (1988) and Skolozdra et al. (1988c). The composition of the alloys, which have been annealed at 673 K, is presented in fig. 13. 

Structure types of ternary stannides obtained by inserting a third component into the structure of binary... 

According to Krypyakevich s classification of the intermetallic compounds on the coordination polyhedra of the atom with the smaller size (Krypyakevich 1977), the structure types of the stannides belong to six classes rhombododecahedral, the class with... 

Structure types of ternary stannides wich consist of fragments of other structures ... 

The next most frequent composition is RSn2. It has been reported that there are other stannides between this composition and the RSn compounds (see table 1), but crystal structure were not determined exeept for La3Sns, Ce3Sn5 and Pt3Sn5 (Pu3Pds type). 

In the R-M-Sn systems, where M = Co, Ru, Rh, Os, Ir, single crystals of the stannides have been synthesized from the melt of the tin (Cooper 1980, Espinosa 1980, Espinosa et al. 1980, 1982). The obtained compounds are very interesting because among some of them there is superconductivity and magnetic ordering. According to the X-ray data of Cooper (1980) there are five types of structures indicated as phases I, II, III, V VII. 

As we have seen from the previous part, the structures of the stannides are connected in some way with the structures of other groups of compounds. Analysis shows, that the structures of the ternary stannides can be divided into three main groups. The first group contains the stannides obtained by an insertion of the third component in the structure of binary compounds. These types are presented in table 6. 

The stannides with the structures of the binary compounds and their superstructures form the second group (table 7). The formation of the superstructures of the ternary stannides is connected with the change of the number of the components and ordered placement of their atoms in Wyckoff sites which are occupied in the binary compounds by one component. In most cases, M and Sn are the ordered atoms, and only in the superstructures of the TiAs type (RPt2Sn stannides) and CrasCg (phases III) do the R and Sn atoms order. The structure of the RNi2Sn (MnCu2Al type) compound can be considered as both a phase of insertion and as a superstructure. 

It follows from eq. (3) that the thermopower must change linearly with the temperature. In most ternary stannides the dependence S T) is complex. Analysis carried out by Skolozdra (1993) shows that the deviations from linearity mainly correlate with the ones in the p(T) dependence and are caused by the same type of mechanisms of the electron scattering. The main causes of the anomalous 5(T) are the peculiarities of the structure of the electron band spectrum in the region of the Fermi level, a strong electron-phonon interaction, and the presence of a statistical distribution of atoms in the crystal structure. 

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