Stannides crystal structure

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. 

The series of equiatomic ytterbium transition-metal stannides has most intensively been investigated. Today twelve stannides with a variety of six different crystal structures are... 

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. 

Phase diagrams of the binary R-Sn systems, the crystal structure and thermodynamic properties of binary stannides have been summarized by Bulanova and Sydorko (1994). 

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). 

The isothermal section of the phase diagram of the Sc-Cu-Sn system at 673 K was constructed (Kotur and Derkach 1994). The system is characterized by the existence of three ternary stannides ScCu4Sn, ScCuSn and Sc6CuSn2. The crystal structure was studied for ScCu4Sn and ScCuSn. The homogeneity range of the ternary compounds is small. 

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 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 resistivities of the ternary stannides have different values and temperature dependences because of the variety of their composition and crystal structures. Skolozdra (1993) classified the stannides by their resistivity behaviours from the curvature of the p(T) function in the range 80-360 K. A tendency that stannides with large M contents have a bigger curvature has been found. A comparison of the influence of different metals (Fe, Co, Ni, Cu) on curvature indicated that as the transition of M from iron to copper occurs, the curvature of the p(f) function decreases and all practically studied copper stannides have a linear p(T) dependence. 

An explanation of the temperature dependence p(T) for stannides is based on two models. According to them, the curvature of the p(T) function is determined either by the band structure in the region of the Fermi level (Mott 1964), or it is caused by the unordering of the atoms in the crystal structure which results in the mean free path of the... 

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. 

The disagreement found for the compounds magnesium silicide and magnesium stannide may be explained as the result of the deformation of the anions (mole refraction of Si 4, 950 of Sn 4, 228) or, on the other hand, the crystals may not have the ionic structure assumed. 

Crystals of the intermetallic compound magnesium stannide, MgjSn, have been prepared and investigated by means of Laue and spectral photographs with the aid of the theory of space-groups. The intermetallic compound has been found to have the calcium fluoride structure, with dwo = 6.78 0.02 A. U. The closest approach of tin and magnesium atoms is 2.94 0.01 A. U. 

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. 

Most plumbides are available only in the form of microcrystalline powders. In contrast to the situation for stannides, it is difficult to grow small single crystals suitable for X-ray investigation. Furthermore the plumbides do not resist the humidity of the air and must therefore be kept under inert conditions. This chemical behavior is similar to the europium-based plumbides (Pottgen and Johrendt 2000). The structural details of the YbrPb phases are given in table 8. 

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. 

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 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). 

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