A BRILLOUIN STUDY OF THE FERROELASTIC PHASE-TRANSITION IN (CH3NH3)(3)SB2BR9

Brillouin spectra of single crystals of (CH3NH3)(3)Sb2Br9 have been me asured as a function of temperature across its ferroelastic transition temperature. An anomalous temperature dependence of the peak paramete rs of the [0 1 0] transverse acoustic mode was observed in the vicinit y of the ferroelastic transition at 160 K. In analogy with (CH3NH3)(3) Bi2Br9, this phase transition is most likely induced by the E-g repres entation at the Gamma-point of the room-temperature phase. The more pr onounced softening of the C-66 elastic stiffness constant observed for (CH3NH3)(3)Sb2Br9, relative to (CH3NH3)(3)Bi2Br9, is attributed to th e higher relaxation frequency of the order parameters in the former. ( C) 1998 Elsevier Science Ltd. All rights reserved.