Formation of Ternary Germanides

For a better demonstration of the influence of the R component on the formation of ternary germanides and their ciystal chemistry, and for the improvement and simplification of the presentation of the crystallographic data of the compoimds, we have characterised the composition of a given phase according to its R/(M,Ge) ratio, and arranged the structure types according to increasing rare-earth content in the next section of this chapter. 

An analysis of the interaction of the components in the ternary systems R-R -Ge for which the isothermal sections have been constructed showed that they can be divided into three groups (a) systems which are characterized by formation of ternary compounds (b) systems which are characterized by formation of continuous solid solutions and limited solid solutions between binary germanides (c) systems in which both ternary compounds and continuous and limited solid solutions are formed. 

Carbides, Silicides, and Germanides. The standard heat of formation and the dissociation energy of TiCjCg) have been determined as — 730 9 and 160 + 8 kJ mol , respectively, using the mass spectrometric Knudsen effusion technique. The standard heat and entropy of formation of TijSijis) have been reported as — 580 kJ moP and 272 e.u., respectively. A thermodynamic analysis of the co-reduction of TiOj with Si02 by carbon at elevated temperatures has shown that the formation of TiSi is more probable than TiSij. Phase equilibria in the Ti-Nb-Ge ternary system have been investigated. ... 

No ternary phase diagram is available for the Sc-Ir-Ge system however, the formation of two ternary scandium iridium germanides has been reported so far. 

No phase diagram is available at present for the La-Fe-Ge system the formation of three ternary germanides has been reported by various authors. 

A partial isothermal section of the Nd-Au-Ge system at 870 K is shown in fig. 73, after Salamakha (1997). The phase relations are characterized by the formation of five ternary neodymium germanides of gold. Their crystallographic data are listed in table 26. For the details of sample preparation, see Nd-Ag-Ge. 

At present, the phase relations are unknown for the system Sc-Pr-Ge. The formation of two ternary germanides was observed in the course of searching for isostructural series of compounds. 

At present, phase relations are unknown for the system Ce-Er-Ge the formation of one ternary germanide was observed. Shpyrka (1990) reported the crystallographic characteristics for Ceo.8Ero.2Ge CrB-type structure, a = 0.4470, b= 1.1104, c = 0.4072 (X-r powder diffraction). For experimental details, see Y-Ce-Ge. 

Although the isothermal sections are completely or partly constructed only for 18 ternary systems from 96 possible R-platinum metals-Ge combinations, one can see that with respect to the formation of compounds in the ternary R-M-Ge systems, Ru and Os show behavior similar to Rh and Ir (see table 37). Analysis of the compositions and crystal structures of the experimentally observed ternary germanides of platinum metals established a likeness between platimun- and palladium-containing systems (see table 38). Since information on the systematic investigations of the R-Pt-Ge systems is still lacking, only some regularities on the interaction of these components are observed. 

Considering the crystal structures of the compounds it is possible to conclude that the presence of scandium leads to the formation of various kinds of superstructures to the structure types of the binary compounds (Sm5Gc4, Ca2As). RScGe ternary germanides are isostructural and are formed with the elements of the cerium subgroup (La, Ce, Pr, Nd, Sm, Eu), i.e. at the ratio r /rsc 1.1 for R=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu the compounds of this composition are not observed. 

Formation of superstructures of the binary rare-earth germanides The crystal structure of Ho26Pd4(Pd,Ge)i9 jc represents a substitutional variant of the Er26Ge23-j structure type. The difference between the two structures lies only in the replacement of germanium atoms by palladium atoms in positions 2(c) and 8(j) and the presence of a statistical distribution of Ce and Pd atoms in positions 2(c) and 8(i). Other isotypical ternary phases are not known. Only the compound Ce26Li5Gc23 -y, where the Li atoms occupy the 2(b) and 8(i) positions which are vacant in the previous phases, has a similar structure (table 5). 

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