Glassy thermal conductivity in halide perovskites: The role of chemical bonding inhomogeneity and lone pair electrons.

This article focuses on the investigation of lattice anharmonicity and its impact on thermal transport properties in crystalline halide perovskites Cs₃Sb₂Cl₉ and Cs₃Bi₂Cl₉ synthesized via a wet chemical process. These materials exhibit ultra-low, glass-like thermal conductivity (0.14–0.19 W m⁻¹ K⁻¹) in the temperature range of 303–373 K, attributed to hierarchical chemical bonding combining mixed ionic (Cs–Cl) and covalent (Sb–Cl, Bi–Cl) interactions and the local off-centering (emphanisis) of stereochemically active 5s² and 6s² lone pair electrons on Sb and Bi. The resulting large lattice anharmonicity leads to a soft lattice with low sound velocity and Debye temperature, suppressing heat-carrying transverse acoustic phonon modes and enhancing phonon scattering. These findings elucidate the fundamental correlation between chemical bonding, lone pair electron effects, and phonon transport, highlighting the potential of such halide perovskites for thermoelectric applications requiring thermally insulating crystalline solids.