Dynamic Local Order and Ultralow Thermal Conductivity of Cs₂AgBiBr₆

Lead halide perovskites are renowned for their exceptional optoelectronic properties but face concerns over lead toxicity and stability, which drives the exploration of lead-free perovskites, with Cs₂AgBiBr₆ standing out as a benchmark alternative. Understanding the structural dynamics and thermal transport properties of Cs₂AgBiBr₆ is crucial but remains an outstanding challenge due to the complex atomic fluctuations. Here, through diffuse scattering experiments and simulations, we uncover the underlying dynamic local structure in Cs₂AgBiBr₆, showing a unique two-dimensional spatial correlation. The inelastic X-ray scattering experiments and simulations further confirm the strong anharmonicity and short phonon lifetimes in Cs₂AgBiBr₆. An ultralow thermal conductivity of ∼0.36 W m^-1 K^-1 was measured by the frequency-domain thermoreflectance technique, with abnormal weak temperature dependence (∼ T^-0.7). These results offer new insights into the lattice dynamics of lead-free double perovskites and are critical to understanding the electron-phonon and phonon-phonon couplings for their applications such as optoelectronics.