Landau model for the elastic properties of the ferroelastic crystal Rb4LiH3(SO4)4

Using sound velocity measurements, we report a detailed investigation of the elastic properties of Rb4LiH3(SO4)(4) realized as a function of temperature and pressure. Results are compared to predictions of a phenomenological Landau model. Supported by recent Raman scattering measurements, we assume that the 4 -> 2 structural transformation observed at T-c = 134 K corresponds to a pseudo-proper ferroelastic transition. For the numerical analysis, all coupling parameters are determined using the temperature dependence of the frequency of the soft optical B mode, the temperature dependence of spontaneous strains, and the pressure dependence dT(c)/dP = 191 +/- 2 KGPa(-1) also determined in this work. Our comparison indicates that the 4 -> 2 structural transition in Rb4LiH3(SO4)(4) is fully consistent with predictions derived using our pseudo-proper ferroelastic model. Thus, all data presented in this paper corroborate that the mechanism leading to the structural transition at T-c = 134 K results from the softening of the B optical mode observed at 31 cm(-1). This detailed analysis also refutes the idea that Rb4LiH3(SO4)(4) shows incomplete softening of the soft acoustic mode also associated with that structural transition.