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 oxovanadium hydrogen phosphates, namely VO(HPO4).nH2O (n=2((alpha)), 2((beta)) and 4) and (beta)-NH4(VO2)(HPO4). The (omega)-VOPO4 phase only remains well crystallised at relatively high temperatures. Its crystal structure has been determined ab initio from Xray powder diffraction data collected at 475°C. The cell is tetragonal (space group P42/mmc) with a=4.8552(3)? and c=8.4301(6)?. Starting positional parameters were obtained by direct methods, and the structure was refined using Rietveld profile refinement principles. Disorder of two oxygen sublattices is observed. To rationalise some features of the crystal structure of this new phase, a dynamic model is proposed and discussed.