Strongly anharmonic phonons in all inorganic halide perovskites

Inorganic halide perovskites CsMX₃ (M=Ge, Sn, Pb; X=Cl, Br, I) have attracted significant attention for their excellent performance in photovoltaic, radiation detection and thermoelectric applications. CsMX₃ systems are very soft and anharmonic materials, which directly impacts electron-phonon coupling and carrier relaxation. The resulting ultralow thermal conductivity also creates potential for thermoelectric applications. CsPbBr₃ has achieved 10% solar cell efficiency and outstanding radiation detection performance, with higher stability than organic-inorganic hybrids. CsSnBr₃ is explored as an alternate lead free system. However, the understanding of atomic dynamics and local distortions in CsMX₃ remains limited. We used inelastic neutron scattering, diffuse neutron and x-ray scattering, and first-principles simulations to investigate the strong phonon anharmonicity, 2D correlated fluctuation dynamics, and phase transitions of CsMX₃ systems. Our experiments reveal how soft and anharmonic phonon modes modulate the structure of CsPbBr₃ and CsSnBr₃, directly impacting the electron-phonon interaction and optoelectronic properties. These results provide insights into the role of unusual atomic dynamics and short-ranged structural fluctuations in CsMX₃.