High-Pressure Behavior and Thermal Stability of Water-Bearing TiO2-II Formed by Phase Transition of Natural Rutile

In situ high-pressure Raman spectroscopy (up to 38 GPa) and thermal stability analysis (up to 900 °C) based on Raman and infrared spectroscopy were performed on water-bearing TiO2-II. The samples were synthesized from natural rutile through pressure-induced phase transition experiments. The results demonstrate that at ambient temperature, water-bearing TiO2-II transforms into akaogiite at 12–15 GPa, which is higher than the phase-transition pressure reported in previous studies for anhydrous TiO2-II samples transforming to akaogiite (approximately 10 GPa). Hydroxyl groups at different positions within the TiO2-II structure exhibit distinct thermal stabilities. Although the water-bearing TiO2-II sample underwent partial dehydration between 300 and 400 °C under atmospheric pressure, the majority of the hydroxyl content persisted at higher temperatures (even after the phase transition from TiO2-II to rutile). At ambient pressure, water-bearing TiO2-II transforms to rutile at 500–600 °C, which is significantly lower than the previously reported transition temperature (730 °C) for anhydrous TiO2-II. The partial dehydration of water-bearing TiO2-II during heating may account for the phase transition occurring at a reduced temperature.