Vibrational study of the structural phase transitions in the (CH3ND3)(3)Sb2Br9 (d-MABA) crystals by infrared spectroscopy

Infrared spectra in the region of the internal vibrations of methylammonium cation (4000 and 400 cm(-1)) around the phase transitions at T-c1 = 166 K and T-c2 = 129 K of the deuterated analogue of MABA are presented and discu ssed. A detail analysis has been performed for the bands assigned to the st retching and deformation vibrations of the ND3+ groups. The selfdiffusion t heory related to the order-disorder phase transitions has been applied for the analysis of bandwidth of some modes below the 129 K phase transition (w ithin the phase III). The results confirm the order-disorder type of this t ransition and give information about the energy barrier for a reorientation of the CH3ND3+ cations. (C) 2001 published by Elsevier Science B.V.