Local polar fluctuations in lead halide perovskite crystals

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in both hybrid (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbBr$_{\mathrm{3}})$ and all-inorganic (CsPbBr$_{\mathrm{3}})$ lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations show that head-to-head Cs motion coupled to Br face expansion, on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr$_{\mathrm{3}}$.