THE PHASE-TRANSITIONS BETWEEN H0.27V0.27W0.73O3-CENTER-DOT-1 3H(2)O AND V0.27W0.73O2.865 - AN X-RAY, THERMAL-ANALYSIS, AND HREM STUDY/

A mixed vanadium-tungsten oxide hydrate (H0.27V0.27W0.73O3 . 1/3 H2O) has been synthesized by a soft chemistry method, and the phase transit ions from the hydrate (precursor) to the final product V0.27W0.73O2.86 5 have been studied by thermal analysis, X-ray powder diffraction, and high-resolution electron microscopy (HREM) techniques. Supermetastabl e, metastable, and stable oxides have been observed. H0.27V0.27W0.73O3 . 1/3H(2)O possesses a structure related to WO3 . 1/3H(2)O. Dehydrati on of the precursor leads to a supermetastable phase, H0.27V0.27W0.73O 3, with a structure similar to that of the hydrate. At 350 degrees C t his phase transforms to the metastable H0.27V0.27W0.73O3, with a struc ture isotypic with WO3(hex). The phase transition between the hydrate and H0.27V0.27W0.73O3 is both pseudomorphous and topotactic, Removal o f hydrogen and oxygen from the metastable oxide induces a second phase transition at 500 degrees C to a more stable phase, V0.27W0.73O2.865, with an oxygen-deficient WO3-type structure. The monoclinic symmetry of the latter oxide increases at higher temperature, first to orthorho mbic and then to tetragonal. This transition is not pseudomorphous but of a topotactic nature. The obtained results suggest that the hydroge n content stabilizes the WO3(hex)-related structure of the metastable phase. A hypothetical model based on HREM observations is proposed for the structural transformation H0.27V0.27W0.73O3--> V0.27W0.73O2.865. (C) 1998 Academic Press.