Rare-earth phosphides structure types

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... 

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. 

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

In the review by Kanatzidis et al. (2005), the preparation by the tin-flux method is mentioned also for several ternary phosphides and polyphosphides of rare-earth and transition metals. Typically the components (R metal, T metal, P and Sn in an atomic ratio of about 1 4 20 50) in sealed silica tubes were slowly heated, to avoid violent reactions, up to 800°C, annealed at that temperature for 1 week and slowly (2 K/h) cooled to ambient temperature. The tin-rich matrix was dissolved in diluted hydrochloric acid. The authors described the preparation of compounds corresponding for instance to the formula MeT4P12 (Me = heavy rare-earth metals and Th and U, T = Fe, Ru, etc.) and to the series of phases MeT2P2 (Me is a lanthanide or an actinide and T a late transition metal) having a structure related to the BaAl4 or ThCr2Si2 types. 

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. 

---