Bond Polarizability as a Probe of Local Crystal Fields in Hybrid Lead-Halide Perovskites

A rotating organic cation and a dynamically disorderedsoft inorganiccage are the hallmark features of organic-inorganic lead-halide perovskites.Understanding the interplay between these two subsystems is a challengingproblem, but it is this coupling that is widely conjectured to beresponsible for the unique behavior of photocarriers in these materials.In this work, we use the fact that the polarizability of the organiccation strongly depends on the ambient electrostatic environment toput the molecule forward as a sensitive probe of the local crystalfields inside the lattice cell. We measure the average polarizabilityof the C/N-H bond stretching mode by means of infrared spectroscopy,which allows us to deduce the character of the motion of the cationmolecule, find the magnitude of the local crystal field, and placean estimate on the strength of the hydrogen bond between the hydrogenand halide atoms. Our results pave the way for understanding electricfields in lead-halide perovskites using infrared bond spectroscopy.