Stability of rhombohedral phases in vanadium at high-pressure and high-temperature: first-principles investigations

摘要

The pressure-induced transition of vanadium from BCC to rhombohedralstructures is unique and intriguing among transition metals. In this work, thestability of these phases is revisited by using density functional theory. Atfinite temperatures, a novel transition of rhombohedral phases back to BCCphase induced by thermal electrons is discovered. This reentrant transition isfound not driven by phonons, instead it is the electronic entropy thatstabilizes the latter phase, which is totally out of expectation. Parallel tothis transition, we find a peculiar and strong increase of the shear modulusC44 with increasing temperature. It is counter-intuitive in the sense that itsuggests an unusual harding mechanism of vanadium by temperature. With thesestability analyses, the high-pressure and finite-temperature phase diagram ofvanadium is proposed. Furthermore, the dependence of the stability of RH phaseson the Fermi energy and chemical environment is investigated. The resultsdemonstrate that the position of the Fermi level has a significant impact onthe phase stability, and follows the band-filling argument. Besides the Fermisurface nesting, we find that the localization/delocalization of the d orbitalsalso contributes to the instability of rhombohedral distortions in vanadium.