Brillouin and ultrasonic studies of ferroelastic phase transition in [N(CH3)(4)](2)ZnBr4

The temperature dependence of the elastic stiffness constant c(55) in [N(CH 3)(4)](2)ZnBr4 single crystals has been measured using Brillouin scattering and the ultrasonic pulse echo overlap technique. The elastic constant c(55 ) was found to soften strongly in both the paraelastic and ferroelastic pha ses. Despite the ferroelastic transition being an. order-disorder type, no elastic dispersion was observed in the frequency range below similar to 2 G Hz. Hence the frequency of the susceptibility of the order parameter, induc ing this transition, is expected to be above similar to 2 GHz.