Origin of Ultralow Thermal Conductivity in Metal Halide Perovskites

Resulting from their remarkable structure-propertyrelationships,metal halide perovskites have garnered tremendous attention in recentyears for a plethora of applications. For instance, their ultralowthermal conductivities make them promising candidates for thermoelectricand thermal barrier coating applications. It is widely accepted thatthe "guest" cations inside the metal halide frameworkact as "rattlers", which gives rise to strong intrinsicphonon resistances, thus explaining the structure-propertyrelationship dictating their ultralow thermal conductivities. In contrast,through systematic atomistic simulations, we show that this conventionallyaccepted "rattling" behavior is not the mechanism dictatingthe ultralow thermal conductivities in metal halide perovskites. Instead,we show that the ultralow thermal conductivities in these materialsmainly originate from the strongly anharmonic and mechanically softmetal halide framework. By comparing the thermal transport propertiesof the prototypical fully inorganic CsPbI3 and an emptyPbI(6) framework, we show that the addition of Cs+ ions inside the nanocages leads to an enhancement in thermal conductivitythrough vibrational hardening of the framework. Our extensive spectralenergy density calculations show that the Cs+ ions havewell-defined phase relations with the lattice dynamics of the "host"framework resulting in additional pathways for heat conduction, whichis in disagreement with the description of the individual "rattling"of guests inside the framework that has been widely assumed to dictatetheir ultralow thermal conductivities. Furthermore, we show that anefficient strategy to control the heat transfer efficacy in thesematerials is through the manipulation of the framework anharmonicityachieved via strain and octahedral tilting. Our work provides thefundamental insights into the lattice dynamics that dictate heat transferin these novel materials, which will ultimately help guide their furtheradvancement in the next-generation of electronics such as in thermoelectricand photovoltaic applications.