A detailed picture of atomic structure and dynamics in materials is needed to refine microscopic theories of transport and thermodynamics, to design next-generation energy materials or computing devices. For instance, phonons impact numerous functional behaviors, ranging from structural transitions and optoelectronic properties of metal halide perovskites (MHPs) to ferroelectric properties or transport properties in thermoelectrics  and solid-state electrolytes. Significant progress has been achieved in understanding simple crystal structures, but disorder and complex temporal and spatial correlations of ions remain challenging to probe and rationalize. Isolating spontaneous off-centerings, subunit distortions or tilts, compositional fluctuations, or soft phonon modes – and their couplings – requires careful measurements and detailed modeling.

This presentation will highlight our investigations of the unusual atomic dynamics and structural fluctuations in MHPs through inelastic neutron scattering (INS), inelastic x-ray scattering (IXS), diffuse neutron/x-ray scattering measurements and comprehensive first-principles and machine-learning augmented simulations. Through INS/IXS and diffuse scattering measurements, we found systematic quasi-elastic diffuse scattering rods in reciprocal space in all MHP compounds investigated -- both single and double-perovskite halides-- in their cubic or tetragonal phases, which originate from fluctuating planar domains featuring correlated tilts of halide octahedra. The diffuse rods exhibit a slow, overdamped dynamic response with a characteristic time of a few picoseconds, modulated across Q space, and reflecting the strong lattice anharmonicity of the inorganic framework. These collective dynamic fluctuations are traced to strongly anharmonic phonon modes featuring an overdamped character, for wavevectors along the edges of the Brillouin zone (single perovskite setting). These results offer valuable insights into the unusual anharmonic atomic dynamics and intricate correlated structural distortions in MHPs, which will be critical for rationalizing and further tailoring their thermal and optoelectronic properties.

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