The compound was prepared at 360C, and the orthorhombic unit-cell parameters were found, by means of electron and X-ray diffraction, to be: a = 1.20819, b = 0.86848, and c = 0.87452nm (Pbcn). Rietveld refinement of X-ray powder diffraction data, chemical analysis, and density measurements showed that the basic structure consisted of a 3-dimensional framework of Sc2(WO4)3-type, with NbO6 octahedra and PO4 tetrahedra sharing corners. Both Nb and P vacancies occurred, and the true formula was close to Nb2-xP3-yO12. Comparative spectroscopic studies of the present compound, and of Nb3(NbO)2(PO4)7, confirmed the occurrence of such vacancies; and therefore a large local variation in the bond lengths near to defect sites. The presence of small amounts of H and N in the structure was considered. Upon heating in air, the structure was maintained at temperatures of up to 900C.

J.J.Zah-Letho, A.Verbaere, A.Jouanneaux, F.Taulelle, Y.Piffard, M.Tournoux: Journal of Solid State Chemistry, 1995, 116[2], 335-42