  Single crystals of the methylammonium (MA) lead halides MAPbI3, MAPbBr3, and
MAPbCl3 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 behavior 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 2-dimensional
orthorhombic hydrogen bond layers into a more 3-dimensional arrangement in the
tetragonal phase seems to be an important feature providing deeper insight into
the mechanisms that lead to a free-rotating MA molecule in the inorganic host
structure. The change of the molecules 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.
