Weak Host Interactions Induced Thermal Transport Properties of Metal Halide Perovskites Deviating from the Rattling Model

TL;DR

This study reveals that the thermal transport in certain metal halide perovskites deviates from traditional models due to weak host interactions, and introduces a new spring model to better explain these properties.

Contribution

The paper develops a new spring model that explains thermal transport in $A_2$SnI$_6$ perovskites, challenging the conventional rattling model.

Findings

Low-frequency phonon branches are insensitive to cation mass.

Weak interactions between octahedral structures dominate thermal transport.

The spring model accurately explains experimental observations.

Abstract

The low-frequency phonon branches of metal halide perovskites typically exhibit the characteristic of hardening with the increase of the cation mass, which leads to anomalous thermal transport phenomenon. However, the underlying physical mechanism is not yet understood. Here, we theoretically compare the thermal transport properties of $A_2$SnI$_6$ ($A$=K, Rb, and Cs) perovskites. The thermal transport in perovskites is widely explained using the rattling model, where ``guest'' cations inside the metal halide framework act as ``rattlers'', but this fails to explain the following phenomenon: The low-frequency phonon branch of $A_2$SnI$_6$ perovskites is insensitive to the mass of the $A^+$ cation and strongly correlated with the interaction of the $A^+$ cation with the I$^-$ anion in the octahedral structures. The failure of the rattling model stems mainly from the weak interactions between the octahedral structures. By developing a new spring model, we successfully explain the thermal transport behavior in $A_2$SnI$_6$ perovskites. Our work gives new insights into the thermal transport mechanism in metal halide perovskites, which has a guiding significance for designing extremely low thermal conductivity materials.