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 omega and A15 structures were also reproduced successfully. More interestingly, we observed another phase (space group: Pnma, 62) that is more stable than omega and A15. Its stability is confirmed by its phonon spectra and elastic constants. For omega-Ta, the calculated elastic constants and high-temperature phonon spectra both imply that it is neither mechanically nor dynamically stable. Thus, omega 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)]. (C) 2013 AIP Publishing LLC.