Two-dimensional local correlations of octahedral tilts dictate thermal transport in three-dimensional metal halide perovskites

In pure nonmetallic crystals, the resistance to heat conduction at temperatures around and above room temperature is conventionally accepted to originate from anharmonic phonon-phonon interactions. Herein, we reveal a different mechanism of heat transfer, which cannot be described by the typical phonon gas picture, where local two-dimensional correlations of the octahedral motions lead to a temperature-independent thermal conductivity (above room temperature and approaching the melting temperature) in ${\mathrm{CsPbI}}_{3}$. Our systematic atomistic simulations allow us to relate the unique lattice dynamics to the thermal transport in ${\mathrm{CsPbI}}_{3}$, providing evidence for the indirect (but crucial) role of the ${\mathrm{Cs}}^{+}$ ions in facilitating heat transfer at relatively higher temperatures. This contrasts with the conventionally accepted notion of the ``rattling'' scenario leading to the reduced thermal conductivities in metal halide perovskites, where the decoupled motion of the ${\mathrm{Cs}}^{+}$ ions scatters the acoustic modes of the framework. Rather, we show that the interaction between the ${\mathrm{Cs}}^{+}$ ions and the ${\mathrm{PbI}}_{6}$ framework enables the local two-dimensional spatiotemporal correlations of the octahedra leading to better heat conduction at higher temperatures in these solids. Our work reveals that local spatiotemporal correlations between atoms, beyond the typical phonon gas picture, can dictate thermal transport in the highly anharmonic metal halide perovskites, and could help guide their further advancements in photovoltaic and thermoelectric applications.