Crystal structure of a new polytype in the V-P-O system: is omega-VOPO4 a dynamically stabilised metastable network?

omega -VOPO4 has been prepared by thermal decomposition of different oxovan adium hydrogen phosphates, namely VO(HPO4) nH(2)O (n = 2(alpha), 2(beta) an d 4) and beta -NH4(VO2)(HPO4). The omega -VOPO4 phase only remains well cry stallised at relatively high temperatures. Its crystal structure has been d etermined ab initio from X-ray powder diffraction data collected at 4750C. The cell is tetragonal (space group P4(2)/mmc) with a = 4.8552(3) Angstrom and c = 8.4301(6) Angstrom. Starting positional parameters were obtained by direct methods, and the structure was refined using Rietveld profile refin ement principles. Disorder of two oxygen sublattices is observed. To ration alise some features of the crystal structure of this new phase, a dynamic m odel is proposed and discussed. (C) 2001 Elsevier Science Ltd. All rights r eserved.