Vibrational spectroscopic studies of the ferroelectric $LiRbSO_4$

Lithium rubidium sulphate, $LiRbSO_4$ (LRS), undergoes a sequence of four phase transitions at 166, 185, 202 and 204C. The phase between 202 and 204C is incommensurate. Polarized phonon Raman spectra in the frequency region of $50-1200 {cm}^-1$ are presented to identify the external and internal vibrational modes at room temperature. The internal mode frequencies of the sulphate ions are presented in the temperature region from -150 to 230C covering all the phase transitions. The total integrated areas of the 1, 2 and 4 modes show an anomalous increase across the phase transitions. The frequencies of the symmetric stretching ( 1) and symmetric bending ( 2) modes do not show any changes at the phase transitions, but the width of the 2 mode shows changes across the phase transitions. A small increase in the linewidth of the 2 mode observed in the incommensurate phase is attributed to the influence of the incommensurate modulation wave. A DSC thermogram showed endothermic peaks during heating at all the phase transitions. The IR spectrum recorded at room temperature showed the expected Au and Bu internal modes