Bond polarizability as a probe of local crystal fields in hybrid lead-halide perovskites

A rotating organic cation and a dynamically disordered\nsoft inorganic\ncage are the hallmark features of organic-inorganic lead-halide perovskites.\nUnderstanding the interplay between these two subsystems is a challenging\nproblem, but it is this coupling that is widely conjectured to be\nresponsible for the unique behavior of photocarriers in these materials.\nIn this work, we use the fact that the polarizability of the organic\ncation strongly depends on the ambient electrostatic environment to\nput the molecule forward as a sensitive probe of the local crystal\nfields inside the lattice cell. We measure the average polarizability\nof the C/N–H bond stretching mode by means of infrared spectroscopy,\nwhich allows us to deduce the character of the motion of the cation\nmolecule, find the magnitude of the local crystal field, and place\nan estimate on the strength of the hydrogen bond between the hydrogen\nand halide atoms. Our results pave the way for understanding electric\nfields in lead-halide perovskites using infrared bond spectroscopy.