Unusual lattice dynamics of vanadium under high pressure.

The electronic structures and lattice dynamics of pressure-induced complex phase transitions [bcc --> hR1(110.5 degrees) --> distorted-hR1(108.2 degrees) --> bcc] in vanadium as a function of pressure up to 400 GPa have been investigated with an ab initio method using density functional perturbation theory (DFPT). At ambient pressure, the soft transverse acoustic phonon mode corresponding to Kohn anomaly appears at a wave vector q = 2k(F) along [xi00] Gamma --> H high symmetry direction. The nondegenerate transverse acoustic branches TA(1) on (110) and TA(2) on (001) show an exceptionally large split at high symmetry point N (0.5 0.5 0.0). The lattice dynamical instability starts at a pressure of 62 GPa (V/V(0) = 0.78, where V(0) is experimental volume of bcc-V at ambient conditions), derived by phonon softening that results in phase transition of bcc --> hR1 (alpha = 110.5 degrees). At compression around 130 GPa (V/V(0) = 0.67), the rhombohedral angle of hR1 phase changed to 108.2 degrees, and the electronic structure changed drastically. At even higher pressure, approximately 250 GPa (V/V(0) = 0.57), lattice dynamic calculations show that the bcc structure becomes stable again.