Infrared reflectivity study of the phase transitions in sodium ammonium sulfatedihydrate

Abstract

This work reports a detailed infrared reflectivity investigation of the phase transitions insingle crystals of sodium ammonium sulfate dihydrate (SASD). The polarizedreflectivity spectra allow us to follow the temperature dependence of the polarvibrational modes and detect the critical behaviour of the vibrational parametersthrough the two low temperature structural phase transitions observed in thecompound. The results obtained show that the mechanism of the transitions inSASD is complex, involving a strong coupling between pseudo-spins and phonons.In the paraelectric phase, the driving mechanism of the first phase transition(Tc1 = 95 K) appears to be related to a relaxation with a characteristic frequency that is much lower thanthe phonon frequencies. In the temperature range corresponding to the first ferroelectric phase(Tc1>T>Tc2 = 79 K),the dynamics of the lattice change considerably and the parameters characterizing several vibrationalmodes display anomalous temperature dependences. The second phase transition occurring atTc2 is marked by an important and discontinuous change of the spectral shape, indicating thata considerable lattice distortion is involved.