On the ferroelastic phase transition of LiNH4SO4: a Brillouin scattering study and theoretical modelling

Brillouin spectroscopy was used to study the elastic properties of LiNH 4 SO 4 in the vicinity of the ferroelastic first-order transition occurring at approximately 285 K. Eleven acoustic modes were observed in the temperature range from 273 to 293 K. Their sound velocities and the temperature dependences of the corresponding elastic constants were calculated from the measured frequency shifts. The measured elastic constants exhibited step-like discontinuities at the transition temperature and were interpreted using a symmetry-adapted Landau free-energy expansion. The order parameter for the ferroelectric phase transition at 460 K was chosen to be an infrared-active soft mode; while for the ferroelastic phase transition at 285 K, a two-component soft mode from the X point of the Brillouin zone was chosen. Both of these order parameters are coupled to polarisation and strains.