Defect Structure of the So-Called Niobium-V Diphosphate - Diffraction and 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 formation of Nb4P6O25 as an impurity phase. Chemical analyses lead to a formula NbP1.8O7 for the Nb(V) diphosphate, and X-ray and electron diffraction studies show its basic structure to be that of ZrP2O7. Both the structure and the formula suggest the occurrence of P vacancies. This is supported by the infrared and MAS NMR spectra, which show the existence of short terminal P-O bonds and of niobyl groups NbO. The formula Nb0.4(NbO)0.6(P2O7)0.8(PO4)0.2 accounts well for the results. Variations in the departure from cubic symmetry were observed; they are probably related to variations in defect order. A comparison with other Nb(V) or Ta(V) phosphates is briefly presented.