Structure and dynamic dielectric behaviour of ferroelectric [NH2(CH3)2]3Sb2Br9 (DMABA)w

The crystal structure of DMABA has been determined at 293 and 180 K. It crystallizes in the monoclinic P2(1)/c space group. The structure was refined to R-1 = 0.0466 for 1269 independent reflections at 293 K and R-1 = 0.038 for 1728 reflections at 180 K. DMABA is isomorphous with the chlorine analogue, DMACA. Its anionic sublattice forms corrugated two-dimensional layers in the be plane. There are two crystallographically independent, disordered dimethylammonium cations in the crystal structure. One of them occupies cavities inside the polyanionic layers; the other is located between the layers. The temperature dependence of lattice parameters has been determined between 100 and 290 K. The presence of the ferro-paraelectric phase transition at 164 K has been confirmed. The complex dielectric permittivity of DMABA single crystals in the frequency range 100 Hz-10 GHz between 10 K and 300 K has been measured. The dynamic dielectric behaviour of DMABA is found to be determined by the properties of two independent relaxators: a low frequency one (similar to 1 MHz at T-c) with a critical slowing down of its dynamics and a high frequency one (similar to 5 GHz), which is thermally activated and contributes to the electric permittivity at least from about 80 K.