Order-disorder type of Peierls instability in BaVS3

Lattice dynamics of low-dimensional ${\mathrm{BaVS}}_{3}$ is reported across the metal-insulator Peierls transition occurring at ${T}_{P}=69$ K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density-functional theory calculations. The nondetection of a Kohn anomaly points to a unique situation of an order-disorder Peierls instability with a quasielastic critical scattering which has been fully characterized. These observations are discussed in the scope of a Peierls instability dominated by strong electron-phonon coupling and/or nonadiabatic effects.