High-pressure stability of bcc-vanadium and phase transition to a rhombohedral structure at 200 GPa

Abstract

The structural stability of transition metal vanadium (V) under high pressure is investigated by using an x-ray diffraction technique combined with a laser annealing method. Although the body-centered cubic (bcc) phase of V showed rhombohedral distortion above 30–69 GPa, as reported in previous studies, with increasing pressure at 298 K, the distortion was eliminated at 58, 118, 119, and 189 GPa by laser annealing up to 2200 K; the bcc phase was observed at room temperature after annealing. These results suggested that the bcc phase is stable up to 189 GPa at room temperature, and the rhombohedral phase with a rhombohedral angle α > 109.47° of the primitive cell for the bcc lattice reported in existing studies is a metastable phase induced by non-hydrostatic pressure. Furthermore, after annealing at 242 GPa, a new high-pressure phase appeared and coexisted with the bcc phase. From different experimental runs up to 300 GPa at 298 K, the transition to the new high-pressure phase was also confirmed at approximately 200 GPa, and the high-pressure phase was explained to be a new rhombohedral phase with α < 109.47°. The present results verified existing theoretical predictions that the rhombohedral phase with α smaller than 109.47° occurs in V at pressures higher than 100 GPa. The equation of state of the bcc phase at room temperature is also proposed up to 240 GPa.