STRUCTURAL INVESTIGATIONS OF AMMONIUM VANADIUM DIPHOSPHATES BY X-RAY-POWDER DIFFRACTION

The crystal structures of three ammonium vanadium diphosphates have be en investigated by means of X-ray powder diffraction including ab init io structure determination and Rietveld refinement. The compound NH4VP 2O7 crystallizes in the monoclinic space group P2(1)/c with a = 7.5149 (2) Angstrom, b = 10.0384(3) Angstrom, c = 8.2422(2) Angstrom, beta = 105.988(3)degrees, and Z = 4. The two ammonium vanadyl diphosphates ar e (NH4)(2)VOP2O7, which has a tetragonal unit cell, space group P4bm, a = 8.3039(2) Angstrom, c = 5.7658(2) Angstrom, and Z = 2, and alpha-( NH4)(2)(VO)(3)(P2O7)(2), which has an orthorhombic structure, space gr oup Pnam, a = 17.4973(4) Angstrom, b = 11.3655(3) Angstrom, c = 7.2769 (2) Angstrom, and Z = 4. The first and the third structures consist of a framework of corner-sharing VOx polyhedra and PO4 tetrahedra and in tersecting channels, whereas (NH4)(2)VOP2O7 is arranged in layers buil t-up from VO5 square pyramids and P2O7 groups and the NH4+ cations in between, The three NH4 compounds are isostructural to the equivalent a lkali vanadium diphosphates. (C) 1998 Academic Press.