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

Abstract

High-quality single crystals of perovskite-like (CHNH)BiI 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 P6/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 P2). 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 along the polar axis in the P2 phase. The calculated polarization has almost equal contributions of the methylammonium and Bi lone pair, which are fairly decoupled.

A (CH3NH3)3Bi2I9 Perovskite Based on a Two‐Step Deposition Method: Lead‐Free, Highly Stable, and with Enhanced Photovoltaic Performance

“…The two crystalline phases of the (CH 3 NH 3 ) 3 Bi 2 I 9 has been observed: (i) Szklarz et al . proposed an orthorhombic structure with Cmcm (63) space group and (ii) Jakubas and Sobczyk determined a hexagonal structure with P6 3 /mm (194) space group . In our case, the crystal structure of the (CH 3 NH 3 ) 3 Bi 2 I 9 shows the hexagonal structure which is in consistent with the structure proposed by Jakubas et al …”

Progress on lead-free metal halide perovskites for photovoltaic applications: a review

“…Single crystals can be synthesized via a layered-solution crystallization technique [176, 180], while thin films are obtained from solution-based processing (e.g. spin coating, doctor blading) followed by subsequent thermal annealing at low temperatures [36, 175, 181–184] or via vapor-assisted methods [181].…”

“…spin coating, doctor blading) followed by subsequent thermal annealing at low temperatures [36, 175, 181–184] or via vapor-assisted methods [181]. The (CH 3 NH 3 ) 3 Bi 2 I 9 structure consists of pairs of face-sharing BiI 6 octahedra forming isolated metal halide dimer units of Bi 2 I 9 3− surrounded by randomly disordered CH 3 NH 3 + cations [175, 177, 180, 182]. The bi-octahedral anionic clusters are interconnected via N–H···I hydrogen bonding interactions [179, 182].…”

“…The bi-octahedral anionic clusters are interconnected via N–H···I hydrogen bonding interactions [179, 182]. Dipolar ordering of the organic cation and in-plane ordering of the lone pair of the metal upon cooling is accompanied by phase transitions from a hexagonal crystal structure (space group: P 6 3 / mmc ) at 300 K to a monoclinic crystal structure (space group: C 2/ c ) at 160 K with an additional first-order phase transition at 143 K (monoclinic, space group: P 2 1 ) [180]. …”

“…(CH 3 NH 3 ) 3 Bi 2 I 9 is an environmentally friendly semiconductor with promising stability in ambient atmosphere and under humid conditions [36, 175, 180, 181, 183, 184]. With regard to the electronic band structure, DFT calculations predict an indirect character of the band gap with values of ca.…”