Soft-phonon driven hexagonal-orthorhombic phase transition in BaVS3

The lattice dynamics of low-dimensional BaVS3 is reported across the hexagonal-orthorhombic phase transition occurring at T-S = 250 K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density functional theory calculations. We observed the occurrence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the F points associated with weak Bragg reflections with odd L. Inelastic scattering experiments related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasielastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at T-S, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at T-MI = 70 K.