Dynamic Structural Fluctuations from Strongly Anharmonic Phonons in Metal Halide Perovskites

Besides their photovoltaic performance metal halide perovskites (MHPs) are attracting increased interest for their excellent performance in optoelectronic and radiation detection, as well as thermoelectric conversion applications. MHPs are known to exhibit a soft lattice with large-amplitude atomic fluctuations. While crucial to understand electron-phonon and phonon-phonon couplings, the spatiotemporal correlations of these fluctuations remain largely unknown. We discuss these correlations based on comprehensive neutron and x-ray scattering measurements, complemented with first-principles simulations augmented with machine learning. Our measurements and simulations reveal complex potential energy surfaces, resulting in multiple soft competing phonons that dynamically distort the lattice at finite temperature. These result in 2D dynamic fluctuations of cooperative halide octahedra tilts, as revealed by characteristic diffuse scattering rods. The short-ranged correlations are not static, and their dynamics are probed with inelastic measurements, revealing pervasive overdamped phonon spectra and quasielastic signatures. These results provide new insights into the atomic structure and fluctuations in MHPs, critical to understand their unusual electron-phonon and phonon-phonon couplings, underlying their optoelectronic, thermal transport and thermodynamic properties.