Polar Nature of (CH₃NH₃)₃Bi₂I₉ Perovskite-Like Hybrids

Abstract

High-quality single crystals of perovskite-like (CH 3 NH 3 ) 3 Bi 2 I 9 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 P 6 3 / mmc ) into a monoclinic phase ( C 2/ 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 P 2 1 ). The dielectric constant is markedly higher in the C 2/ 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 P 2 1 phase. The calculated polarization has almost equal contributions of the methylammonium and Bi 3+ lone pair, which are fairly decoupled.