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

TL;DR

This study uses first-principles calculations to explore the stability and phase transitions of vanadium under high pressure and temperature, revealing a novel electronic entropy-driven reentrant transition and unusual temperature effects on mechanical properties.

Contribution

It uncovers a temperature-induced reentrant transition in vanadium driven by electronic entropy and analyzes the influence of Fermi level and d orbital localization on phase stability.

Findings

Reentrant transition from rhombohedral to BCC phase driven by electronic entropy.

Unusual increase of shear modulus C44 with temperature.

Fermi level position and d orbital localization significantly affect phase stability.

Abstract

The pressure-induced transition of vanadium from BCC to rhombohedral structures is unique and intriguing among transition metals. In this work, the stability of these phases is revisited by using density functional theory. At finite temperatures, a novel transition of rhombohedral phases back to BCC phase induced by thermal electrons is discovered. This reentrant transition is found not driven by phonons, instead it is the electronic entropy that stabilizes the latter phase, which is totally out of expectation. Parallel to this transition, we find a peculiar and strong increase of the shear modulus C44 with increasing temperature. It is counter-intuitive in the sense that it suggests an unusual harding mechanism of vanadium by temperature. With these stability analyses, the high-pressure and finite-temperature phase diagram of vanadium is proposed. Furthermore, the dependence of the stability of RH phases on the Fermi energy and chemical environment is investigated. The results demonstrate that the position of the Fermi level has a significant impact on the phase stability, and follows the band-filling argument. Besides the Fermi surface nesting, we find that the localization/delocalization of the d orbitals also contributes to the instability of rhombohedral distortions in vanadium.