Stable structures of tantalum at high temperature and high pressure

Knowledge of the structures of Ta that appear at high pressures and high temperatures is critical for addressing the recent controversies regarding the phase diagram and melting temperatures of this refractory metal. Structural searches based on a density functional metadynamics method were therefore employed to obtain candidate structures at high-pressure and finite-temperature conditions. A structural transformation from the initial body-centered cubic phase of Ta stable under ambient conditions to an orthorhombic structure with the space group Pnma is predicted. The Pnma structure is shown to be energetically more favorable and more reasonable than other candidate structures considered previously, since it was also confirmed to be mechanically and dynamically stable by phonon and metadynamics calculations. However, a recently proposed hexagonal-{ω} phase for dense Ta is found to be mechanically and dynamically unstable when anharmonic effects are characterized by high-temperature, self-consistent phonon calculations.