Parallel Alignment of Methylammonium Cations in an Orthorhombic CH₃NH₃PbCl₃ Single Crystal Observed by Polarized Micro-Raman Scattering Spectroscopy

Abstract

To increase their efficiency and improve their chemical stability, hybrid organic–inorganic perovskites (HOIPs) have been studied intensely over the past few years. There are some unresolved issues with the properties of HOIPs, such as the relation between the configuration of methylammonium (MA) ions and their electronic and structural properties. For example, it was theoretically proposed that the change in the bandgap from direct to indirect and/or structural distortion can be caused by a change in the orientation of the MA ions with respect to the PbX3 cage in CH3NH3PbX3 (X = I, Br, or Cl). By using polarized Raman scattering spectroscopy, we experimentally show that the MA ions in CH3NH3PbCl3 single crystals are parallelly oriented toward a special crystallographic direction in the low-temperature orthorhombic phase at least on our measurement scale of approximately square micrometers. Experimental measurement results verified by density functional theory calculations indicate the presence of a macroscopic domain consists of a parallel directional alignment of the MA ions, a strong signature of ferroelectricity in HOIPs.

Internal Vibrations of Pyridinium Cation in One-Dimensional Halide Perovskites and the Corresponding Halide Salts

“…MA + and FA + cations behave similarly in the high-temperature and intermediate-temperature phases, rotating isotropically and reorienting between their preferred orientations, respectively [ 9 , 14 , 15 , 16 , 17 , 18 , 19 ]. In the low-temperature phase, MA + cations still have an ordered three-fold rotational axis [ 17 , 20 ], while the FA + cations are locally disordered, exhibiting a glass-like state in the arrangement of the cations [ 14 ]. In 2D perovskites, large organic cations have greater freedom of motion compared to 3D perovskites and tend to interact with each other by weak intermolecular forces and with the inorganic framework through strong electrostatic interactions [ 5 , 6 , 21 , 22 , 23 ].…”

“…Organic cations are spectators of the crystalline environment [ 22 , 35 ]. The polarized Raman spectroscopy of organic vibrations could be used to determine the crystal orientation [ 33 ] and the cation movement [ 20 ].…”

Stabilizing Polar Domains in MAPbBr3 via the Hydrostatic Pressure-Induced Liquid Crystal-like Transition

“…The existence of polar nanoregions under ambient conditions has been debated in MAPbI 3 and MAPbCl 3 . 67,68 In MAPbBr 3 , it was shown that surface reconstruction can lead to polar domains at low temperatures, while the bulk remained nonpolar. 69 Here we show that pressure can be a tool for achieving stable polar domains in bulk MAPbBr 3 at room temperature, which in turn can improve photoconversion efficiency 58,59 in addition to influencing other electronic properties.…”