High-pressure behavior of TiO2 as determined by experiment and theory

Using high-resolution synchrotron x-ray powder diffraction we have investigated the structural phase transitions and equations of state of titanium dioxide (TiO2) under high pressure before and after heating at high temperature. The phase sequence we observe experimentally is as follows: rutile (RT)-> columbite (CB)-> baddeleyite (MI)-> orthorhombic I (OI)-> orthorhombic II (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 (+/- 34) GPa and room-pressure volume (V-0) of 25.28 (+/- 0.35) A(3) as compared to previous experiments. We find that the volume decreases across the OI -> OII phase transition at room temperature by similar to 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.