Critical phonon frequency renormalization and dual phonon coexistence in layered Ruddlesden-Popper inorganic perovskites

Phonon-related behaviors such as hot-carrier bottleneck and thermooptic coefficients stimulate the study of lattice dynamics and thermal conductivity (kappa) in perovskites. Herein, we report ultralow thermal conductivities of synthesized highly oriented samples of Cs2PbI2Cl2 (similar to 0.45 W/mK at 300 K) and Cs2SnI2Cl2 (similar to 0.60 W/mK at 300 K). Phonon frequency renormalization of the octahedral rotational soft modes in Cs2PbI2Cl2 triggered by the fourth-order anharmonicity considerably amends the three- and four-phonon linewidths, significantly altering the thermal transport property of Cs2PbI2Cl2. Furthermore, the phonon gas model partially breaks down, and both normal and diffuson-like phonons must be considered to capture the nearly temperature-independent kappa of Cs2PbI2Cl2. We also find these unusual phonon behaviors in other layered inorganic perovskites such as Cs2PbBr2Cl2 and Cs2SnI2Cl2. Our results rationalize the crucial high-order phonon interactions and two-channel lattice thermal transports in perovskites with ultralow kappa.

Automated summary

By studying the lattice dynamics and thermal conductivity in perovskites, we discovered that Cs2PbI2Cl2 and Cs2SnI2Cl2 samples have extremely low thermal conductivities. The phonon frequency renormalization of the octahedral rotational soft modes in Cs2PbI2Cl2 plays a crucial role in the thermal transport properties. We also observed similar phonon behaviors in other layered inorganic perovskites.