Al3+ and H+ substitutions in TiO2 polymorphs: structural and vibrational investigations

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-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-TiO2 is at least one order of magnitude greater than those in rutile- and CaCl2-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 the incorporation of smaller amounts of Al3+ cations postpones the phase transition pressure. The lattice vibrational modes typically shift to lower frequencies at elevated temperature and to 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 mechanism in TiO2 and SiO2 structures.