Brillouin scattering study of the incommensurate transition of K2SeO4

All elastic stiffness constants have been determined at room temperature using Brillouin spectroscopy. Some of them and the spectral width of the corresponding Brillouin peaks show important variations around the transition temperature (Ti = -143°C) between prototypic and incommensurate phases. Below Ti, the anomalies associated with the acoustic phonons propagating in the (001) direction are explained assuming a linear coupling between deformations and an overdamped mode. This mode is equivalent to a Debye relaxation process characterized by a relaxation time τ0 = 2.6 10-12 s.K.