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

Fundamental understanding of the interplay between chemical bonding, lattice dynamics, and phonon transport is crucial for thermoelectric applications, and the desire for thermally insulating crystalline solids has gained notable attention over the years. Anharmonicity prevailing in crystalline solids has a significant effect on how heat moves through the lattice, and its correlation with low thermal conductivity is discussed in this work. Here, we have synthesized crystalline halide perovskites Cs3Sb2Cl9 and Cs3Bi2Cl9 via the wet chemical process and explored the effect of lattice anharmonicity in thermal transport properties, which exhibits glassy thermal conductivity of 0.17–0.19 W m−1 K−1 and 0.14–0.16 W m−1 K−1 in the temperature range of 303–373 K. The hierarchical chemical bonding with mixed ionic and covalent interactions and local off-centering of stereochemical active 5s2 and 6s2 lone pair of electrons on Sb and Bi generates large lattice anharmonicity resulting in soft lattice with low sound velocity (927 and 897 ms−1) and Debye temperature (105 and 102 K). The presence of low frequency localized Cs–Cl bond vibration and the repulsion between the lone pair and the bond pair suppresses the heat carrying transverse acoustic phonon modes to steer glassy thermal conductivity in halide perovskites.