High-Pressure Phase Transitions in Densely Packed Nanocrystallites 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 originates from the different interfacial energies of the involved titania phases, which control their relative thermodynamic phase stabilities at nano sizes. This work enriches our understanding of the complex phase behaviors of titania and provides fundamental knowledge for developing applications of the less-explored titania nanophase of TiO₂-II.