Raman spectra and improper ferroelastic phase transition in LiNH4SO4 single crystal

Polarized Raman spectra of lithium ammonium sulphate (LAS) single crystals in the temperature range 40-290 K have been studied and compared with the a vailable data. Careful analysis of their temperature evolution it made poss ible to propose a comprehensive assignment of the observed Raman active pho non modes and to gain deeper insight into the lattice dynamics of this crys tal in the low-temperature ferroelastic phase III. In particular, numerous new modes connected with doubling of the unit cell were unambiguously ident ified in agreement with group-theoretical predictions. The peculiar tempera ture evolution of the spectra below the II --> III transition temperature T -2 (284 K) is explained by coexistence of both phases, with metastable pola r clusters and heterophase fluctuations persisting in a wide temperature ra nge below T-2. This interpretation accounts also for other anomalies observ ed in macroscopic properties of LAS in this temperature interval.