  We report a strongly temperature dependent luminescence Stokes shift in the
electronic spectra of both hybrid and inorganic lead-bromide perovskite single
crystals. This behavior stands in stark contrast to that exhibited by more
conventional crystalline semiconductors. We correlate the electronic spectra
with the anti-Stokes and Stokes Raman vibrational spectra. Dielectric solvation
theories, originally developed for excited molecules dissolved in polar
liquids, reproduce our experimental observations. Our approach, which invokes a
classical Debye-like relaxation process, captures the dielectric response
originating from an anharmonic LO phonon at about 20 meV (160 cm-1) in the
lead-bromide framework. We reconcile the liquid-like picture with more standard
solid-state theories of the Stokes shift in crystalline semiconductors.
