Predicted alternative structure for tantalum metal under high pressure and high temperature

First-principles simulations have been performed to investigate the phase stability of tantalum (Ta) metal under high pressure and high temperature. We searched its low-energy structures globally using our developed multi-algorithm collaborative crystal structure prediction technique. The body-centered cubic (bcc) was found to be stable at pressure up to 300 GPa. The previously reported ω and A15 structures were also reproduced successfully. More interestingly, we observed another phase (space group: Pnma, 62) that is more stable than ω and A15. Its stability is confirmed by its phonon spectra and elastic constants. For ω-Ta, the calculated elastic constants and high-temperature phonon spectra both imply that it is neither mechanically nor dynamically stable. Thus, ω is not the structure to which bcc-Ta transits before melting. On the contrary, the good agreement of Pnma-Ta shear sound velocities with experiment suggests Pnma is the new structure of Ta implied by the discontinuation of shear sound velocities in recent shock experiment [J. Appl. Phys. 111, 033511 (2012)].