Infrared Spectroscopic Study of Vibrational Modes across the Orthorhombic-Tetragonal Phase Transition in Methylammonium Lead Halide Single Crystals

Single crystals of the methylammonium (MA) lead halides MAPbI(3), MAPbBr(3), and MAPbCl(3) have been investigated using infrared spectroscopy with the aim of analyzing structural and dynamical aspects of processes that enable the ordering of the MA molecule in the orthorhombic crystal structure of these hybrid perovskites. Our temperature-dependent studies were focused on the analysis of the CH/NH rocking, C-N stretching, and CH/NH bending modes of the MA molecule in the 800-1750 cm(-1) frequency range. They deliver a direct comparison of the behaviors of the three halides on crossing the orthorhombic-tetragonal phase transition in MA lead halide single crystals. Drastic changes of all vibrational modes close to the phase transition were clearly observed. Additional spectral features that were not discussed previously are pointed out. The transformation of the two-dimensional orthorhombic hydrogen bond layers into a more three-dimensional arrangement in the tetragonal phase seems to be an important feature providing deeper insights into the mechanisms that lead to a free-rotating MA molecule in the inorganic host structure. The change of the molecule site symmetry in the tetragonal crystal structure seems to be an important feature of the orthorhombic-tetragonal phase transition. For low temperatures, it can be stated that the iodide is stronger influenced by hydrogen bonding than the bromide and the chloride.