Phosphides structure types

Three hypothetical ternary structures of the R/T, M), c family derived from known binary transition metal boride and phosphide structure types. 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

Compounds pertaining to this structural types are phosphides and arsenides of Sr and Eu and Ba3P4. Several R5Sb2X2 compounds (with R = Y or a lanthanide and X = Si or Ge) have the Tm5Sb2Si2-type structure which is an ordered superstructure (of the second kind) of the oS36-Eu5As4 type and corresponds to the following data ... 

Still further distortions can take place. In NiP, the chains of Ni and P atoms discernible in the MnP structure break up into Ni2 and P2 pairs. For phosphides, it is experimentally clear that the number of available electrons tunes the transition from one structural type to another. Nine or 10 valence electrons favor the NiAs structure (for phosphides), 11-14 the MnP, and a greater number of electrons the NiP alternative. For the arsenides this trend is less clear. 

Metal arsenides, antimonides and bismuthides can be prepared by direct combination of the metal and group 15 element. Like the phosphides, classification is not simple, and structure types vary. Our coverage here is, therefore, selective... 

The phosphide is bright yellow, the arsenide is brown-red. Stable in completely dry air at room temperature, decompose in moist air (MgsPs decomposes faster than MggAsg). Both crystallize in structure type D5g (carbon sesquioxide). d (MgsPs ) 2.055 (MgsAsg) 3.148. 

Lattice parameters, structure types and residuals for the phosphides YbrP ( Pd, Pt)... 

Among the remarkably diversified variety of binary and ternary meta boride structures, the triangular prismatic coordination MgB has the dominating role, whereas among metal phosphides, sillcides and carbides this is merely true for a limited series of structure types. 

The ThCfjSij-type, a BaAl4-type derivative, shown in fig. 50a, is the most frequent structure type found in R-T-B, R-T-Si and R-T-Ge systems. It is also found with phosphides and related compounds. For a recent literature survey see Jeitschko and Jaberg (1980). 

Krypyakevych (1977) has proposed the systematics of structure types of intermetallic and related compounds based on the analysis of the coordination polyhedra of the smallest atom. This author distinguishes 17 groups to which the coordination polyhedra of the smallest atoms belong. In analyzing the crystal structures of intermetallic compounds some problems, connected with choice of the smallest atom, arise. In the case of the metal compounds with boron, carbon, silicon, and phosphorus the nonmetal element is always the smallest. Kuz ma (1983) successfully used this concept for the analysis of the structure types of borides. In analyzing the structure types of the binary and ternary phosphides we will hold to the systematics developed by Krypyakevych (1977) also because the CPs of phosphorus atoms are given in the figures. 

The reported data on the formation of binary rare-earth phosphides and their crystal structure provides an opportunity to make some generalizations. The data about compositions and structure types of all known binary rare-earth phosphides are listed in table 16. 

Phosphorus CPs in the structures of the binary rare-earth phosphides belong to six groups and the structure types belong to four of 17 groups distinguished by Krypyakevych (1977). The smallest atoms have only a few kinds of coordination polyhedra. The relationship of such a structure to a definite structure group is determined ly the polyhedron with the fewest number of vertices. As one can see from table 17 there is... 

Some information about the valence state of the lanthanoid atoms in their compounds shows the dependence of the lattice parameters due to the change of the R atomic number. All the rare earths may be considered for monophosphides with the NaCl-type structure (fig. 20). Using the listed data one can make conclusions about the R valence state of all the rare earths. A small deviation from a linear dependence of the lattice parameter vs. atomic number is observed only for cerium monophosphide. This anomaly is probably due to an intermediate valence of cerium atoms between Ce + and Ce" ". For the light rare-earth pentaphosphides we consider the change of the unit-cell volumes which correspond to one formula units. The variation is caused by the RP5 phosphides belonging to two structure types with a different number of formula units per unit cell. As one can see (fig. 20b) a linear dependence indicates the trivalent state of the R atoms in these compounds. 

The structure types of the ternary and quaternary phosphides are presented in order of increasing phosphorus content and at the same P content in order of increasing rare-earth content. Some closely related STs are also discussed. Information about the structure types of ternary phosphides is reported similarly as for binary phosphides above. 

Classification of structure types of ternary and quaternary phosphides... 

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