The Phase Transitions between H 0.13V 0.13Mo 0.87O 3·0.26H 2O and MoO 3: An X-Ray, Thermal Analysis, and TEM Study

Mixed vanadium-molybdenum oxide hydrates (H _xV _xMo _{1- x}O ₃·0.26H ₂O with 0.06≤ x≤0.18) have been synthesized by a soft chemistry method. The phase transitions from one of these hydrates ( x=0.13) to the final product MoO ₃have been studied by thermal analysis, X-ray powder diffraction, and transmission electron microscopy techniques. Both metastable and stable oxides have been observed. H _0.13V _0.13Mo _0.87O ₃·0.26H ₂O possesses a structure related to hexagonal MoO ₃. Dehydration of the precursor leads to a metastable phase H _0.13V _0.13Mo _0.87O ₃, with a structure similar to that of the hydrate. At 500°C this phase transforms into the metastable V _0.13Mo _0.87O _2.935, with a structure related to the orthorhombic MoO ₃structure. The heating of this last phase above 500°C induces a change from the metastable system to the stable binary (V ₂O ₅-MoO ₃) one. In agreement with this binary, a liquid phase and a solid phase with a composition closest to MoO ₃, are formed during the phase segregation at 600°C. Models for explaining these phase transitions are proposed.