Aluminum incorporation into TiO2 has been studied in the TiO2-Al2O3 system as a function of pressure at temperatures of 900 and 1300 °C using commercial Al2TiO5 nanopowder as starting material. A new orthorhombic TiO2 polymorph with the CaCl2 structure has been observed in the recovered samples synthesized from 4.5 to 7 GPa and 900 °C and from 2.5 to 7 GPa at 1300 °C. The phase transition to the α-PbO2 type TiO2 phase takes place between 7 and 10 GPa at both temperatures. Two mechanisms of Al incorporation in TiO2 rutile have been observed in the recovered samples. The substitution of Ti4+ by Al3+ on normal octahedral sites is dominant at lower pressures. High pressure induces the incorporation of Al3+ into octahedral interstices of the rutile structure, which is responsible for an orthorhombic distortion of the TiO2 rutile structure and gives rise to a (110) twinned CaCl2 type structure. This phase is probably a result of temperature quench at high pressure. Aluminum solubility in TiO2 increases with increasing pressure. TiO2 is able to accommodate up to 9.8 wt% Al2O3 at 7 GPa and 1300 °C. Temperature has a large effect on the aluminum incorporation in TiO2, especially at higher pressures. High pressure has a strong effect on both the chemistry and the microstructure of Al-doped TiO2. Enhanced aluminum concentration in TiO2 rutile as well as TiO2 grains with a microstructure consisting of twins are a clear indication of high-pressure conditions.

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