Soft-phonon driven hexagonal-orthorhombic phase transition in BaVS 3

International audience; The lattice dynamics of low-dimensional BaVS3 is reported accross the hexagonal-orthorhombic phase transition occuring at TS = 250 K using a combination of Thermal Diffuse Scattering of X-rays, Inelastic X-ray Scattering and DFT calculations. We observed the occurence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the Γ points associated with weak Bragg reflections with odd L. Inelastic scattering experiment related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasi-elastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at TS, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at T MI = 70 K.