Parallel Alignment of Methylammonium Cations in an Orthorhombic CH₃NH₃PbCl₃Single Crystal Observed by Polarized Micro-Raman Scattering Spectroscopy

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.