High-Pressure Phase Transitions in Densely Packed Nano-Crystallites of TiO2-II

Phase behaviors of nanocrystalline TiO2-II have not been well studied due to the difficulty in its preparations at ambient pressure. In this work, we prepared nanocrystallites of TiO2-II with average sizes of ∼10–20 nm via high-pressure processing of anatase particles in a large volume press. X-ray diffraction (XRD) and electron microscopy examinations show that the TiO2-II nanocrystallites formed from cracking and phase transformation of large anatase particles under compression. The high-pressure phase behaviors of the TiO2-II nanocrystallites were investigated using in situ synchrotron XRD and Raman spectroscopy. Results show that, under compression up to ∼35–40 GPa, the phase transitions in nano TiO2-II proceed via two parallel routes of TiO2-II (∼9–15 GPa) → baddeleyite (∼25 GPa) → TiO2-OI and TiO2-II (∼19 GPa) → TiO2-OI. At different TiO2-II sizes, the TiO2-II-to-baddeleyite transition occurs at pressures from ∼9 to 15 GPa, while the TiO2-II-to-TiO2-OI transition occurs at ∼19 GPa. This difference o...