  In the last decade, hybrid organic-inorganic halide perovskites have emerged
as a new type of semiconductor for photovoltaics and other optoelectronic
applications. Unlike standard, tetrahedrally bonded semiconductors (e.g. Si and
GaAs), the ionic thermal fluctuations in the halide perovskites (i.e.
structural dynamics) are strongly coupled to the electronic dynamics.
Therefore, it is crucial to obtain accurate and detailed knowledge about the
nature of atomic motions within the crystal. This has proved to be challenging
due to low thermal stability and the complex, temperature dependent structural
phase sequence of the halide perovskites. Here, these challenges are overcome
and a detailed analysis of the mode symmetries is provided in the
low-temperature orthorhombic phase of methylammonium-lead iodide. Raman
measurements using linearly- and circularly- polarized light at 1.16 eV
excitation are combined with density functional perturbation theory (DFPT). By
performing an iterative analysis of Raman polarization-orientation dependence
and DFPT mode analysis, the crystal orientation is determined. Subsequently,
accounting for birefringence effects detected using circularly polarized light
excitation, the symmetries of all the observed Raman-active modes at 10 K are
assigned.
