Crystal structure, phase transition and ferroelectric properties of the [(CH3)3NH]3[Sb2Cl9(1−x)Br9x](TMACBA) mixed crystals

Abstract

The paraelectric–ferroelectric phase transition in the[(CH3)3NH]3[Sb2Cl9(1−x)Br9x] (TMACBA) mixed crystals is investigated by the differential scanning calorimetry,dilatometric and dielectric methods. The phase transition is found at363.5, 362.7, 360.5 and 350.9 K (on cooling) for TMACBA crystals withx = 0,0.02, 0.17 and 0.42, respectively. The crystal structures of the pure bromine analogue(x = 1),TMABA, and the mixed crystal TMACBA(x = 0.55)are determined at 297 K. TMABA crystallizes in the trigonalspace group R3̄c: a = 15.098(2) Å, c = 21.906(4) Å,Z = 3,R1 = 0.0393,wR2 = 0.0746.Its structure is built up of discrete Sb2Br93−bioctahedra and disordered trimethylammonium cations. Ferroelectric TMACBA(x = 0.55)crystallizes in the monoclinic space groupPc: a = 10.245(2) Å, b = 9.122(2) Å, c = 15.549(3) Å, β = 90.52(3)°,Z = 2,R1 = 0.0340,wR2 = 0.0712.The crystal is built of two-dimensional layer anions (‘honeycomb-like structure’)and three trimethylammonium cations. Dielectric studies on TMACBA(x = 0.42)in the frequency range 75 kHz–900 MHz indicate relatively fast reorientation of thetrimethylammonium cations over the paraelectric phase. The ferroelectricity ofmixed crystals is preserved for Br concentration smaller than 0.60 ± 0.05.The polar properties are strictly connected with the presence of the polyanioniclayer structure. A possible mechanism for the paraelectric–ferroelectric phasetransition in the TMACBA mixed crystals is discussed on the basis of the resultspresented.