Soft phonon and low-temperature phase transitions in Rb2ZnBr4

The soft phonon modes and the Bragg reflections were measured by inelastic and elastic neutron scattering techniques in order to characterize the phase transitions at T-3 = 112 K and T-4 = 76 K. Two low-energy modes were observed above T-3. One is a soft mode, whose frequency at S point (1/21/20) vanishes at T-3, and the other remains hard at the phase transitions. Below T-4 new Bragg reflections appeared and the intensity increased continuously down to 10 K. The space group below T-4 is suggested to be Clc1 with Z = 48 from the extinction rule.