The Phase Transitions between H0.13V0.13Mo0.87O3·0.26H2O and MoO3: An X-Ray, Thermal Analysis, and TEM Study

Mixed vanadium-molybdenum oxide hydrates (HxVxMo1-xO3·0.26H2O with 0.06x0.18) have been synthesized by a soft chemistry method. The phase transitions from one of these hydrates (x=0.13) to the final product MoO3have been studied by thermal analysis, X-ray powder diffraction, and transmission electron microscopy techniques. Both metastable and stable oxides have been observed. H0.13V0.13Mo0.87O3·0.26H2O possesses a structure related to hexagonal MoO3. Dehydration of the precursor leads to a metastable phase H0.13V0.13Mo0.87O3, with a structure similar to that of the hydrate. At 500°C this phase transforms into the metastable V0.13Mo0.87O2.935, with a structure related to the orthorhombic MoO3structure. The heating of this last phase above 500°C induces a change from the metastable system to the stable binary (V2O5–MoO3) one. In agreement with this binary, a liquid phase and a solid phase with a composition closest to MoO3, are formed during the phase segregation at 600°C. Models for explaining these phase transitions are proposed.