Phase Transition in (CH₃NH₃)₃Bi₂Br₉Single Crystal

Abstract

The successive phase transitions in (CH 3 NH 3 ) 3 Bi 2 Br 9 (MABB) single crystal have been investigated by measuring of dielectric and elastic dispersion and Brillouin and Raman scattering. Softening of the elastic constant c 66 and critical slowing down of the order parameter, which belongs to the E g representation, have been found in the phase I. No optical soft mode has been observed. The phase transition from the phase I to the phase II has been discussed by the Landau theory.