The transposition structure of NbAs and of similar monophosphides and arsenides of niobium and tantalum

Abstract

NbAs crystallizes in a new structure type, which is a transposition structure variation of the WC structure type. NbAs is tetragonal with a = 3.452 A and c = 11.679 A. The space group is I41md (C~) with 4Nb in 4a with z = 0 and 4As in 4a with z =~. Other isotypic compounds are TaAs with a = 3.43~/~ and c = 11.656/~ and 'fl-NbP' with a = 3.334 A and c = 11.376/~. The previously proposed structure for a and fl-NbP is proved to be wrong. Stoichiometric NbP or 'fl-NbP' crystallizes in the NbAs structure, but P-deficient :NbPl-z has a partly disordered NbAs structure. The transposition is not as regular as in stoichiometric NbP. The degree of disorder depends nearly linearly on phosphorus deficiency. The new structure type is compared with other known monophosphide structure types. The concept of a structural transposition and its cancellation offers a new viewpoint for an explanation of the occurrence of more than one crystal structure for the same compound.