Zhong-Li Liu, Ling-Cang Cai, Xiu-Lu Zhang, Feng Xi
First-principles simulations have been performed to investigate the phase stability of tantalum under the extreme conditions of high pressure and high temperature (HPHT). In order to find the stable and alternative metastable structures of tantalum under compression, we globally searched its low-energy structures using our developed multi-algorithm collaborative crystal structure prediction (MACCSP) technique. We found that at 0 K body-centred cubic (bcc) is the unique stable structure at pressure up to 300 GPa. Furthermore, we observed a metastable phase (space group: Pnma, 62) who has competitive energy with respective to bcc and has much lower energy than previously reported \omega and A15 phases. Its stability are confirmed by both its phonon spectra and elastic constants. While, as for \omega -Ta, the calculated elastic constants and high-temperature phonon spectra imply that it is neither mechanically nor dynamically stable at HPHT. The shear sound velocities of Pnma-Ta suggest that Pnma is very likely to be the structure that was implied by the discontinuation of shear sound velocity in recent shock experiment [J. Appl. Phys. 111 (2012) 033511], and further suggest it is the phase to which bcc transforms before melting.
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http://arxiv.org/abs/1306.0266
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