High-pressure behavior ofTiO2as determined by experiment and theory

Abstract

Using high-resolution synchrotron x-ray powder diffraction we have investigated the structural phase transitions and equations of state of titanium dioxide $({\text{TiO}}_{2})$ under high pressure before and after heating at high temperature. The phase sequence we observe experimentally is as follows: rutile $(\text{RT})\ensuremath{\rightarrow}\text{columbite}\text{ }(\text{CB})\ensuremath{\rightarrow}\text{baddeleyite}\text{ }(\text{MI})\ensuremath{\rightarrow}\text{orthorhombic}\text{ }\text{I}\text{ }(\text{OI})\ensuremath{\rightarrow}\text{orthorhombic}\text{ }\text{II}\text{ }(\text{OII})$. The equations of state as determined from our experiments are consistent with previous measurements and computations. The only exception is the OII phase for which we find a significantly lower room-pressure bulk modulus $({K}_{0})$ of 312 $(\ifmmode\pm\else\textpm\fi{}34)\text{ }\text{GPa}$ and room-pressure volume $({V}_{0})$ of 25.28 $(\ifmmode\pm\else\textpm\fi{}0.35)\text{ }{\text{\AA{}}}^{3}$ as compared to previous experiments. We find that the volume decreases across the $\text{OI}\ensuremath{\rightarrow}\text{OII}$ phase transition at room temperature by $\ensuremath{\sim}8.3%$, in very good agreement with our static first-principles calculations which predict volume changes of 8.2% and 7.6% for local-density approximation and generalized gradient approximation, respectively. This volume collapse is significantly higher than previously determined but consistent with the volume decrease observed in other dioxides across this transition.