Al³ and H substitutions in TiO₂ polymorphs: Structural and vibrational investigations.

Wang, Sha; Wang, Qingbo; Ye, Yu; Liu, Dan; Zhu, Xi; Hu, Yancheng; Miao, Yunfan; Wu, Zhen; Pan, Yanming

Rutile is the most common TiO2 mineral on Earth's surface and transforms to CaCl2- and α-PbO2-type structures at elevated pressures in subducted basaltic crusts. In this study, we synthesized hydrous CaCl2- and α-PbO2-type TiO2 crystals with various Al3 concentrations using a multi-anvil press. Al3 is incorporated into the CaCl2- and rutile-type phases mainly in the form of 3Ti4 = 4Al3 , while the coupled substitution of Ti4 =Al3 H is dominant in the α-PbO2-type structure, forming Ti1−x(AlH)xO2 solid solutions. Consequently, the water solubility in Al-bearing α-PbO2-type TiO2 is at least one order of magnitude greater than those in rutile- and CaCl2-type TiO2 phases, making TiO2 a significant water carrier at the pressure-temperature (P-T) conditions in the mantle transition zone (410 to 660 km depth in deep Earth's interior), when coexisting with Al3 and Fe3 . High-P and high-T Raman spectra were collected for these synthetic samples. The CaCl2- and α-PbO2-type phases irreversibly transform to a rutile-type structure at 950 K and ambient pressure. A reversible α-PbO2→baddeleyite phase transition in TiO2 is detected at approximately P = 10 GPa and T = 300 K, and incorporating smaller amounts of Al3 cations increases the phase transition pressure. The lattice vibrational modes typically shift to lower frequencies at elevated temperature and higher frequencies with increasing pressure due to variations in Ti(Al)-O bond length with temperature or pressure. Fourier transform infrared (FTIR) spectroscopic measurements were conducted on the samples under high-T or high-P conditions. Both T- and P-dependences are negative for the OH stretching vibrations in these TiO2 polymorphs, except that the OH bands in the α-PbO2-type samples exhibit a blueshift at elevated temperature. A negative linear correlation can be drawn between the measured OH stretching frequencies and the incorporated M3 O6 quadratic elongation, which were computed based on first-principles calculations. The local octahedral distortion can provide useful insights for understanding the M3 and H incorporation mechanisms in TiO2 and SiO2 structures.

INTERNAL structure of the Earth; REVERSIBLE phase transitions; PHASE transitions; SURFACE of the earth; HIGH temperatures

American Mineralogist, 2025, Vol 110, Issue 2, p241

0003-004X

Academic Journal

10.2138/am-2024-9316