Pressure-induced bcc-rhombohedral phase transition in vanadium metal

Abstract

Vanadium is reported to undergo a pressure-induced bcc-rhombohedral phase transition at 30-70 GPa, with a transition pressure that is sensitive to the hydrostaticity of the sample environment. However, the experimental evidence for the structure of the high-pressure phase being rhombohedral is surprisingly weak. We have restudied vanadium under pressure to 154 GPa using both polycrystalline and single-crystal samples, and a variety of different pressure transmitting media (PTM). We find that only when using single-crystal samples does one observe a rhombohedral high-pressure phase; the high-pressure diffraction profiles from the polycrystalline samples do not fit a rhombohedral lattice, irrespective of the PTM used. The single-crystal samples reveal two rhombohedral phases, with a continuous transition between them, and distortions from cubic symmetry are much smaller than previously calculated.

Pressure-induced structural phase transition of vanadium: A revisit from the perspective of ensemble theory

“…which are all in a good agreement with our results, in considerations of the deviations among the measured pressures being of ∼ 2 GPa. It is noticeable that the RH 2 → BCC → RH 1 transition sequence, neither observed in earlier experiments nor proposed by previous computations, was observed in the one of the latest experiments[20], and the measured transition pressure of RH 2 , 40 GPa, coincides with our theoretical one, 44 GPa. When the pressure is increased larger than 132 GPa, our results indicate that RH 1 would exist until RH 2 transition occurs at 154 GPa and the RH 2 phase keeps stable with the pressure up to 254 GPa, over which the BCC may coexist with RH 2 until the pressure increases up to 280 GPa when BCC becomes the most stable for higher pressures.…”

“…Akahama et al [19] compressed foil V up to 300 GPa at room temperature and found that BCC lattice is a stable phase until the RH 2 and BCC phase coexisting at pressure (P ) larger than 242 GPa, while the RH 1 phase is a metastable phase caused by nonhydrostatic pressure effects. Stevenson et al [20], on the other hand, observed that the BCC lattice transform into RH 2 when P > 40 GPa and then turns back to BCC until RH ), which is significantly different from the theoretical result, 110.5 • (or 108.5 • ) for RH 1 (or RH 2 ).…”

“…BYN is grateful to A. Daniele, M. McMahon and Y. Akahama for their kindly providing the raw experimental data in Refs. [11], [19] and [20], respectively. Part of the computational tasks was conducted in HPC platform supported by The Major Science and Technology Infrastructure Project of Material Genome Big-science Facilities Platform supported by Municipal Development and Reform Commission of Shenzhen.…”

“…The above mentioned phase transition was further confirmed by later experiments [7][8][9][10][11], and the relevant theoretical works, based on either approximate calculations of free energy [12,13] or ab initio lattice dynamics [14][15][16][17][18], reached a qualitative agreement that the transition pressure (P c ) would be 60 ∼ 90 GPa for BCC→RH 1 , and predicted that P c for RH 1 → RH 2 and RH 2 → BCC are around 120 and 250 GPa respectively. Nevertheless, two experiments reported in year 2021 [19,20] exhibited different results. Akahama et al [19] compressed foil V up to 300 GPa at room temperature and found that BCC lattice is a stable phase until the RH 2 and BCC phase coexisting at pressure (P ) larger than 242 GPa, while the RH 1 phase is a metastable phase caused by nonhydrostatic pressure effects.…”

“…These results show clearly, based on the Gibbs FE criterion, that the phase transition with the angle equal to or larger than 0.2% cannot take place unless the deviation being smaller, and furthermore, Fig.3(b)indicates that the two phases, RH 1 with a deviation of 0.1% and RH 2 with a deviation of −0.1%, would emerge when the pressure is larger than 20 GPa. It is interesting to note that the phase transition was very recently examined by Stevenson et al[20] who concluded that two kinds of phase transitions, BCC→ RH 1 and BCC → RH 2 , indeed take place, qualitatively coinciding with the previous experiments or theories, and the determined RH angle for RH 1 (or RH 2 ) is 109.54 • (or 109.35 • ), corresponding to |∆| ∼ 0.1% as our theoretical results, which is a little smaller than the one, 109.65 • for RH 1 , measured in previous experiment…”