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

Abstract

First-principles simulations have been performed to investigate the phase stability of tantalum metal under high pressure and high temperature (HPHT). We searched its low-energy structures globally using our developed multi-algorithm collaborative (MAC) crystal structure prediction technique. The body-centred 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)].

Efficient wide-range calculation of free energies in solids and liquids using reversible-scaling molecular dynamics

“…Furthermore, the MGPT-MD simulations directly showed that hex-ω exhibits poor mechanical stability and partial transformation to bcc, suggesting that it would actually be mechanically unstable at all temperatures and pressures in an unconstrained environment. The mechanical instability of hex-ω has since been so confirmed with first-principles, finite-temperature latticedynamics calculations [17,18]. Thus, of the structures studied in Refs.…”

“…12 as well as to the orthorhombic Pnma phase suggested by Yao and Klug [17] and by Liu et al [18]. In addition to treatment with MGPT potentials, the present free-energy method can also be adapted to other temperature-independent potentials, including GPT, BOP, EAM, and FS potentials among others, for suitable applications in relevant materials.…”

“…12 and 16, only A15, fcc and hcp remain as candidate metastable phases for our RSMD free-energy method. To that list additional candidate structures may be added in the future, including the orthorhombic Pnma structure, which recently has been predicted to be mechanically stable in Ta over wide ranges of temperature and pressure [17,18].…”

Melting curves and structural properties of tantalum from the modified-Z method

“…The melting points are obtained by pursuing the steady solid-liquid coexistence phase. 24) at high pressure and high temperature is controversial, bcc phase has been proved to be the most stable phase at T ¼ 0 up to 300 GPa. In molecular dynamics simulation, it is really a hard work to choose a proper potential to describe the phase relation of Ta.…”

“…On one hand, whether or not a solid-solid phase transition occurred before melting is a hot topic. Several possible phases, i.e., hex-x, [18][19][20][21] A15, 22,23 and pnma, 24 have been reported to be the possible competitive phases with bodycentered cubic (bcc) at high pressure and high temperature. However, no agreement has been reached among them at the moment.…”