Chromium incorporation into TiO2 at high pressure

Chromium incorporation into TiO2 up to 3 GPa at 1300 °C and 900 °C has been studied by XRD as well as TEM. A CaCl2 type TiO2 polymorph has been observed in the quenched samples from high pressure. Two different mechanisms of solubility occur in the recovered samples. Chromium replaces titanium on normal octahedral sites but it also occupies interstitial octahedral sites, especially in the samples recovered from higher pressures. Interstitial chromium is responsible for an orthorhombic distortion of the TiO2 rutile structure in the quenched samples and gives rise to a (1 1 0) twinned CaCl2-structured polymorph. This phase is very likely the result of temperature quench at high pressure. The formation of this phase is directly related to the chromium content of the TiO2 grains. Chromium solubility in TiO2 increases with increasing the synthesis pressure. TiO2 is able to accommodate up to 15.3 wt% Cr2O3 at 3 GPa and 1300 °C, compared to 5.7 wt% at atmospheric pressure at the same temperature. - Graphical abstract: Microstructure consisting of twins domains of recovered Cr-doped CaCl2 type TiO2 grains synthesised at high pressure. Highlights: ► Chromium solubility in TiO2 increases at high pressure. ► Chromium occupies substitutional and interstitial positions in the rutile structure. ► Interstitial chromium causes a decrease of the rutile symmetry. ► An orthorhombic CaCl2 type structure is observed in the quenched samples.