Phase Relations of Ni2In-Type and CaC2-Type Structures Relative to Fe2P-Type Structure of Titania at High Pressure: A Comparative Study

Density functional theory (DFT) based on first-principles calculations was used to study the high-pressure phase stability of various phases of titanium dioxide (TiO2) at extreme pressures. We explored the phase relations among the following phases: the experimentally identified nine-fold hexagonal Fe2P-type phase, the previously predicted ten-fold tetragonal CaC2-type phase of TiO2, and the recently proposed eleven-fold hexagonal Ni2In-type phase of the similar dioxides zirconia (ZrO2) and hafnia (HfO2). Our calculations, using the generalized gradient approximation (GGA), predicted the Fe2P → Ni2In transition to occur at 564 GPa and Fe2P → CaC2 at 664 GPa. These transitions were deeply investigated with reference to the volume reduction, coordination number decrease, and band gap narrowing to better determine the favorable post-Fe2P phase. Furthermore, it was found that both transitions are mostly driven by the volume reduction across transitions in comparison with the small contribution of the electronic energy gain. Additionally, our computed Birch–Murnaghan equation of state for the three phases reveals that CaC2 is the densest phase, while Ni2In is the most compressible phase.