X-RAY INVESTIGATION OF MECHANISM OF FERRO-PARAELECTRIC PHASE TRANSITION AT 164K IN [(CH3)2NH2]3Sb2Br9 (DMABA)
Jacek Zaleski
Institute
of Chemistry, University of Opole, 45-951 Opole, Oleska 48,
Poland,
Email Zaleski@uni.opole.pl
Keywords: Phase Transition, Mechanism, Feroelectric
[(CH3)2NH2]3Sb2Br9 (DMABA) belongs to the group of akylammonium halogenoantimonates (III). This family of compounds, recently attracts much attention, since many salts of this group undergo numerous phase transitions some of them to polar phases [1 and ref. therein]. They are molecular ionic salts. The anionic sublattices are composed of distorted SbCl63- octahedra isolated or connected with each other on many different ways, depending on stoichiometry. Alkylammonium cations are located in voids of anionic sublattice. At room temperature, most crystals are characterised by significant freedom of reorientations of cations. On lowering temperature the cationic motions are successively frozen. Most mechanisms of phase transitions, up to date, was associated with freezing of overall reorientations of cations.
DMABA was reported to undergo two phase transitions below room temperature [2]. One of second order at 228 K to the intermediate phase and second one of mixed order at 164 K to the ferroelectric phase.
The crystal structure of DMABA was determined at 300, 180, 150 and 100 K. Presence of phase transition at 164 K was confirmed. The mechanism of this transition will be discussed. It was confirmed that the mechanism is associated with ordering of one of crystallographically non-equivalent cations. At the phase transition temperature the anionic sublattice undergo significant deformation. This is suggested to take place as a result of electrostatic deformation of Sb-Br bond lengths caused by cation-anion interaction.
We could not find any
evidence of suggested phase transition at 228 K.