Brillouin scattering study of ferroelectric NH4LiSO4: Relaxation time and anomalous temperature dependence of elastic constants

A relaxation time of the order parameter in the ferroelectric phase of ammonium lithium sulfate has been determined by analyzing anomalous temperature dependence of the Brillouin peak intensity. It is shown that the anomalies of the elastic constants associated with the phase transition cannot be interpreted by a conventional phenomenological theory and that the effect of nonlinear elasticity dominates in the ferroelectric phase.