NB2-XP3-YO12, A NOVEL NB-V OXOPHOSPHATE WITH DISORDERED CATION VACANCIES IN A SC-2(WO4)(3)-TYPE STRUCTURE

The title compound was prepared at 360 degrees C, starting from Nb2O5 and NH4H2PO4. The orthorhombic unit-cell parameters were determined by electron and X ray diffractions: a = 12.0819 (2) Angstrom, b = 8.6848 (1) Angstrom c = 8.7452 (1) Angstrom, space group Pbcn. Rietveld refi nements from X ray powder diffraction data, chemical analyses, and the measured density show that the basic structure consists of a three di mensional framework of the Sc-2(WO4)(3) structure type, with NbO6 octa hedra and PO4 tetrahedra sharing corners, and that both Nh and P vacan cies occur. The actual formula is close to Nb2-xP3-yO12. Comparative s pectroscopic studies of the title compound and of Nb-3(NbO)(2)(PO4)(7) confirm the occurrence of the vacancies, and thus an important local variation of the bond lengths near the defect sites. The presence of s mall amounts of H and N in the structure is discussed. Upon heating in air, the structure is preserved up to 900 degrees C. The so-called ep silon-NbOPO4 has the same basic structure, and its existence seems que stionable. (C) 1995 Academic Press, Inc.