Raman study of the phase transition in RbHSO 4

Raman spectra of RbHSO 4 have been recorded in the temperature range from 6 K to the melting point, 480 K. Analysis of the spectra show that the second order phase transition, at 264 K, Is of the order‐disorder type. No soft mode was found. The evolution of the ν(OH) mode frequency with temperature, is consistent with a change of orientation of the hydrogen bonds connecting the disordered SO 3 O(H) groups. The spectra are also consistent with the existence of a prototype phase which had been assumed to explain the pseudo‐orthorhombic symmetry of the paraelectric and ferroelectric phases.