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 available data. Careful analysis of their temperature evolution it made possible to propose a comprehensive assignment of the observed Raman active phonon modes and to gain deeper insight into the lattice dynamics of this crystal in the low-temperature ferroelastic phase III. In particular, numerous new modes connected with doubling of the unit cell were unambiguously identified in agreement with group-theoretical predictions. The peculiar temperature evolution of the spectra below the II --> III transition temperature T-2 (284 K) is explained by coexistence of both phases, with metastable polar clusters and heterophase fluctuations persisting in a wide temperature range below T-2. This interpretation accounts also for other anomalies observed in macroscopic properties of LAS in this temperature interval.