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Single crystals of the methylammonium (MA) lead halides MAPbI3, MAPbBr3, andMAPbCl3 have been investigated using infrared spectroscopy with the aim ofanalyzing structural and dynamical aspects of processes that enable theordering of the MA molecule in the orthorhombic crystal structure of thesehybrid perovskites. Our temperature-dependent studies were focused on theanalysis of the CH/NH rocking, C-N stretching, and CH/NH bending modes of theMA molecule in the 800-1750 cm-1 frequency range. They deliver a directcomparison of the behavior of the three halides on crossing theorthorhombic-tetragonal phase transition in MA lead halide single crystals.Drastic changes of all vibrational modes close to the phase transition wereclearly observed. Additional spectral features that were not discussedpreviously are pointed out. The transformation of the 2-dimensionalorthorhombic hydrogen bond layers into a more 3-dimensional arrangement in thetetragonal phase seems to be an important feature providing deeper insight intothe mechanisms that lead to a free-rotating MA molecule in the inorganic hoststructure. The change of the molecules site symmetry in the tetragonal crystalstructure seems to be an important feature of the orthorhombic-tetragonal phasetransition. For low temperatures it can be stated that the iodide is strongerinfluenced by hydrogen bonding than the bromide and the chloride.

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