High-quality single crystals of perovskite-like (CH3NH3)3Bi2I9 hybrids have been synthesized, using a layered-solution crystal-growth technique. The large dielectric constant is strongly affected by the polar ordering of its constituents. Progressive dipolar ordering of the methylammonium cation upon cooling below 300 K gradually converts the hexagonal structure (space group P63/mmc) into a monoclinic phase (C2/c) at 160 K. A well-pronounced, ferrielectric phase transition at 143 K is governed by in-plane ordering of the bismuth lone pair that breaks inversion symmetry and results in a polar phase (space group P21). The dielectric constant is markedly higher in the C2/c phase above this transition. Here, the bismuth lone pair is disordered in-plane, allowing the polarizability to be substantially enhanced. Density functional theory calculations estimate a large ferroelectric polarization of 7.94 μC/cm(2) along the polar axis in the P21 phase. The calculated polarization has almost equal contributions of the methylammonium and Bi(3+) lone pair, which are fairly decoupled.
Kamminga, M. E., Stroppa, A., Picozzi, S., Chislov, M., Zvereva, I. A., Baas, J., ... Palstra, T. T. M. (2016). Polar Nature of (CH3NH3)3Bi2I9 Perovskite-Like Hybrids. Inorganic chemistry, 56(1), 33-41. https://doi.org/10.1021/acs.inorgchem.6b01699