Raman spectroscopic study of the phase transitions and pseudospin phonon coupling in sodium ammonium sulphate dihydrate

Abstract

This work reports a detailed study of the temperature dependence of the lattice dynamics of single crystal sodium ammonium sulphate dihydrate (SASD) obtained from polarized Raman scattering. The phase transitions in SASD are marked by the appearance of new weak bands and anomalies in the temperature dependence of wave number and damping factor of several lattice and internal modes. The temperature dependence of the symmetric bending modes of ammonium ion as well as the abrupt shift of wave number of some lattice and internal modes related to the molecular units point to a proton ordering mechanism associated with the paraelectric-ferroelectric phase transition. The anomalies, which have been detected in the temperature dependence of the wave number and of the damping coefficient of some external and internal modes, are well described by a theoretical model that takes into account the coupling between pseudospins and phonons.