"Nb 2- xP 3- yO 12," a Novel Nb V Oxophosphate with Disordered Cation Vacancies in a Sc 2(WO 4) 3-Type Structure

Abstract

The title compound was prepared at 360°C, starting from Nb 2O 5 and NH 4H 2PO 4. The orthorhombic unit-cell parameters were determined by electron and X ray diffractions: a = 12.0819 (2) Å, b = 8.6848 (1) Å, c = 8.7452 (1) Å, space group Pbcn. Rietveld refinements from X ray powder diffraction data, chemical analyses, and the measured density show that the basic structure consists of a three-dimensional framework of the Sc 2(WO 4) 3 structure type, with NbO 6 octahedra and PO 4 tetrahedra sharing corners, and that both Nb and P vacancies occur. The actual formula is close to Nb 2- x P 3- y O 12. Comparative spectroscopic studies of the title compound and of Nb 3(NbO) 2(PO 4) 7 confirm the occurrence of the vacancies, and thus an important local variation of the bond lengths near the defect sites. The presence of small amounts of H and N in the structure is discussed. Upon heating in air, the structure is preserved up to 900°C. The so-called ε-NbOPO 4 has the same basic structure, and its existence seems questionable.