Dynamic Structural Fluctuations and Strongly Anharmonic Phonons in Inorganic Halide Perovskites

Inorganic metal halide perovskites (MHPs) CsMX₃(M= Sn, Pb; X=Cl, Br, I) as well as double-perovskite variants are attracting significant attention for their excellent performance in photovoltaic, optoelectronic and radiation detection devices, as well as potential for thermoelectric applications. MHPs are known to exhibit a soft lattice with large atomic fluctuations. While crucial to understand electron-phonon and phonon-phonon couplings, the spatio-temporal 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 phonons that dynamically distort the lattice at finite temperature, forming 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 and thermal properties.