Pressure-induced bcc-rhombohedral phase transition in vanadium metal
M.G. Stevenson, Edward J. Pace, C. V. Storm, S. E. Finnegan, Gastón Garbarino, Craig Wilson, D. McGonegle, Simon G. MacLeod, M. I. McMahon
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.