Microscopic Mechanism of Lattice Thermal Transport in All-Inorganic Perovskites CsSnBr3 and CsCaBr3: A First-Principles Study

Abstract

Inorganic halide perovskites have been extensively studied in optoelectronic, photovoltaic, and thermoelectric applications because of their excellent electronic and thermal transport properties. However, understanding their lattice dynamics and heat transport physics remains challenging due to their inherent lattice instability and strong anharmonicity. In this work, we studied the effects of cubic and quartic anharmonicity on lattice dynamics and thermal transport in highly anharmonic CsCaBr3 and CsSnBr3 crystal across a temperature range of 300–1000 K. Combining first-principles based self-consistent phonon theory with a unified thermal transport theory accounting for both populations’ and coherences’ contribution, we find that anharmonic phonon renormalization including both bubble and loop diagram is essential for accurately describing phonon energies at elevated temperature. At room temperature, the calculated lattice thermal conductivity κL is ∼0.54 for CsCaBr3 and ∼0.33 W m–1 K–1 for CsSnBr3. The remarkably low κL arises from the strong suppression of acoustic phonons by highly anharmonic optical modes, including flat phonon modes dominated by Cs atoms and soft modes dominated by Br atoms. This pronounced anharmonicity stems primarily from weak bonding in the crystal structure, particularly between Cs and other constituent atoms. Additionally, the contribution of the particle-like propagation dominates total thermal conductivity across the entire temperature range, while wave-like tunneling contributions remain non-negligible, especially at high temperatures. Our findings elucidate the underlying mechanisms of lattice dynamics in CsCaBr3 and CsSnBr3, providing valuable insights for designing advanced optoelectronic and thermoelectric materials based on inorganic halide perovskites.