Crystal structure, phase transition and ferroelectric properties of the [(CH₃)₃NH]₃[Sb₂Cl_9(1−x)Br_9x] (TMACBA) mixed crystals

The paraelectric–ferroelectric phase transition in the [(CH₃)₃NH]₃[Sb₂Cl_9(1−x) Br_9x]  (TMACBA) mixed crystals is investigated by the differential scanning calorimetry, dilatometric and dielectric methods. The phase transition is found at 363.5, 362.7, 360.5 and 350.9 K (on cooling) for TMACBA crystals with x = 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 trigonal space group Rc: 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 group Pc: 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 the trimethylammonium cations over the paraelectric phase. The ferroelectricity of mixed crystals is preserved for Br concentration smaller than 0.60 ± 0.05. The polar properties are strictly connected with the presence of the polyanionic layer structure. A possible mechanism for the paraelectric–ferroelectric phase transition in the TMACBA mixed crystals is discussed on the basis of the results presented.