Vibrational spectroscopic studies of the ferroelectric LiRbSO₄

Abstract

Lithium rubidium sulphate, LiRbSO₄ (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 v₁, v₂ and v₄ modes show an anomalous increase across the phase transitions. The frequencies of the symmetric stretching (v₁) and symmetric bending (v₂) modes do not show any changes at the phase transitions, but the width of the v₂ mode shows changes across the phase transitions. A small increase in the linewidth of the v₂ 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.