Phase transitions in rutile-related V _0.92 O ₂ synthesized at high pressures and temperatures

Structural phase transitions in metastable rutile-related V_0.92O₂ synthesized at 10 GPa and 1273 K were studied with single-crystal X-ray diffraction in the temperature range 110-500 K and at pressures up to 9.2 GPa. V_0.92O₂ starts to decompose at 470 K and atmospheric pressure. When heated to temperatures above about 350 K, the material transforms to an incommensurate phase. The oxygen sublattice is essentially rigid and it is mostly the V atoms that are affected by the modulation. An anharmonic description of the displacement parameters and their corresponding modulation is used for the V atoms to reach satisfactory agreement factors for main and first-order satellite reflections, indicating substantial disorder in the modulated structure. Measurements of electronic transport properties provide evidence that the incommensurate phase is insulating. On compression at room temperature, V_0.92O₂ reaches the ideal rutile structure at about 5.0 GPa. Both structural and electronic phase transitions are of the first-order character. The results of this work demonstrate that the structural and electronic behaviour of V_0.92O₂ at extreme conditions is distinctly different from that of stoichiometric VO₂.