Rare-earth phosphides

The present chapter consists of two main sections, presenting data about binary and ternary rare-earth phosphides, respectively. Section 1 deals with material on phase diagrams of R-P systems, the crystal structures of all known binary phosphides, their chemical properties, and methods of preparation. Section 2 contains similar information on ternary rare-earth phosphides, and additionally includes systematized data on physical properties and the valence state of lanthanoid atoms. 

The crystallochemical peculiarities and structural relationships of the binary and ternary rare-earth phosphides are discussed from the viewpoint of the coordination of the phosphorus atoms. 

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. 

Known scandium phosphides contain 50 or less at.% of phosphorus and in this w they differ in principle from other rare-earth phosphides. The crystal structures of scandium phosphides (except ScP) are characterized by a trigonal-prismatic coordination of phosphorus atoms. They are more closely related to the structures of the IVa transition-metal (Zr and Hf) phosphides than with the other rare-earth phosphides. 

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... 

Composition and stmcture types of binary rare-earth phosphides... 

The chemical properties of the binary rare-earth phosphides have been studied fully only for monophosphides, their thermodynamic characteristics have also been determined. Thermodynamic data of formation and atomization of the R monophosphides are listed in table 19. 

Powders of the monophosphides (except EuP and, to some extent, LaP) are stable in air. Mironov et al. (1977) determined that rare-earth phosphides are oxidized in air above 673 K. The products of oxidation are complex phosphates and above 1223 K orthophosphates. 

In summary we can conclude that the chemical stability of binary rare-earth phosphides increases with increasing phosphorus content of the compounds. 

The ST PbFCl (Nieuwenkamp and Bijvoet 1931) is a superstructure of the ST Cu2Sb (Westgren et al. 1929). Since no atomic parameters have been reported for ternary rare-earth phosphides we used data from the CeFeSi structure. The smallest atoms (P) occupy positions in the centers of Archimedean cubes formed by metal atoms. Known phosphides with the PbFCl-type structure are SmFeP, RCoP (R=Pr, Nd, Sm) and LaRuP. 

The peculiarities of a number of STs of ternary rare-earth phosphides require a critical examination of the data on their crystal structure. A great number of them are determined by the powder method which does not permit distinction between such small structure details. 

All known structures of the ternary rare-earth phosphides with a phosphorus content close to 33.3 at.% (except YbCu3P2) are characterized by a metallic trigonal-prismatic arrangement of P atoms. Besides, these compounds have a number of common structure attributes ... 

The structure relationships of ternary rare-earth phosphides with metal-to-nonmetal ratio 2 1 are shown in fig. 83. 

Decreasing phosphorus content in ternary rare-earth phosphides results in two possible changes in the structures ... 

Fig. 87. Structural relations of (a) AlB2-type and (b) La203-type related ternary rare-earth phosphides.

Fig. 88. Stacking oflayers of Archimedean cubes in ternary rare-earth phosphides with BaA -related structures.

Physical properties of ternary rare-earth phosphides and the valence state of the rare-earth atoms... 

Systematic investigation of the chemical properties of the ternary rare-earth phosphides has not been carried out. Some authors report chemical properties of some compounds. 

Mironov, K.E., 1988, Peeuliarities of formation of rare earth phosphides at metals saturation by phosphorus and homogeneity ranges of rare earth monophosphides, in Novelty in Obtaining and Utilization of Phosphides and Phosphorous Containing Alloys, ed. A.M. Kounayev (Nauka, Alma Ata) pp. 67-69. In Russian,... 

Mironov, K.E., l.G. Vasil eva and G.P. Brygalina, 1977, Oxidation of rare earth phosphides, in Semiconductive Materials and their Utilization (Voronezh State University) pp. 111-122. In Russian. 

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