  A rotating organic cation and a dynamically disordered soft inorganic cage
are the hallmark features of hybrid organic-inorganic lead-halide perovskites.
Understanding the interplay between these two subsystems is a challenging
problem but it is this coupling that is widely conjectured to be responsible
for the unique behavior of photo-carriers in these materials. In this work, we
use the fact that the polarizability of the organic cation strongly depends on
the ambient electrostatic environment to put the molecule forward as a
sensitive probe of local crystal fields inside the lattice cell. We measure the
average polarizability of the C/N--H bond stretching mode by means of infrared
spectroscopy, which allows us to deduce the character of the motion of the
cation molecule, find the magnitude of the local crystal field and place an
estimate on the strength of the hydrogen bond between the hydrogen and halide
atoms. Our results pave the way for understanding electric fields in
lead-halide perovskites using infrared bond spectroscopy.
