The ultralow thermal conductivity of Cs2SnI6 by filling interstitial voids with Mg2+ ions

Metal halide perovskites (MHPs) have received considerable attention due to their intrinsically disordered structures, leading to abnormally low thermal conductivity, which is beneficial for energy harvesting applications. Compared with organic–inorganic hybrid MHPs, all inorganic MHPs exhibit better thermal stability, but thermal conductivity is relatively high, about twice that of hybrid MHPs. In this study, we investigate an all-inorganic vacancy-ordered double perovskite, Cs2SnI6, characterized by a cage-like structure with an octahedral interstitial radius of approximately 1.638 Å, allowing for the accommodation of various metal ions. We successfully introduced Mg2+ ions as guest fillers into the octahedral interstices of Cs2SnI6. The weak ionic bonds formed by Mg2+ in these oversized interstices induce vibrations that enhance atomic disorder and anharmonicity within the structure. As a result, we achieved an exceptionally low thermal conductivity of κ = 0.11 W m−1 K−1 at room temperature in Mg0.102Cs2SnI6, representing a 21% reduction compared to the undoped sample and marking one of the lowest values reported for MHPs. This research offers a novel approach to effectively regulate disorder and significantly reduce the thermal conductivity of all-inorganic MHPs.