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 204°C. The phase between 202 and 204°C 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 230°C covering all the phase transitions. The total integrated areas of the ν 1 , ν 2 and ν 4 modes show an anomalous increase across the phase transitions. The frequencies of the symmetric stretching (ν 1 ) and symmetric bending (ν 2 ) modes do not show any changes at the phase transitions, but the width of the ν 2 mode shows changes across the phase transitions. A small increase in the linewidth of the ν 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 A u and B u internal modes.