DEFECT STRUCTURE OF THE SO-CALLED NIOBIUM-V DIPHOSPHATE - DIFFRACTIONAND SPECTROSCOPIC STUDIES

The Nb(V) diphosphate is obtained by heating mixtures of NH4H2PO4 and Nb2O5 at 650-950-degrees-C. It is difficult, however, to avoid the for mation of Nb4P6O25 as an impurity phase. Chemical analyses lead to a f ormula NbP1.8O7 for the Nb(V) diphosphate, and X-ray and electron diff raction studies show its basic structure to be that of ZrP2O7. Both th e structure and the formula suggest the occurrence of P vacancies. Thi s is supported by the infrared and MAS NMR spectra, which show the exi stence of short terminal P-O bonds and of niobyl groups NbO. The formu la Nb0.4(NbO)0.6(P2O7)0.8(PO4)0.2 accounts well for the results. Varia tions in the departure from cubic symmetry were observed; they are pro bably related to variations in defect order. A comparison with other N b(V) or Ta(V) phosphates is briefly presented.