Impact of strain-insensitive low-frequency phonon modes on lattice thermal transport in A2XB6 -type perovskites

Substrate induces mechanical strain on perovskite devices, which can result in alterations to its lattice dynamics and thermal transport. Herein, we have performed a theoretical investigation on the anharmonic lattice dynamics and thermal property of perovskite Rb₂SnBr₆ and Cs₂SnBr₆ under strains using perturbation theory up to the fourth-order terms and the unified thermal transport theory. We demonstrate a pronounced hardening of low-frequency optical phonons as temperature increases, indicating strong lattice anharmonicity and the necessity of adopting temperature-dependent interatomic force constants in the lattice thermal conductivity (κL) calculations. It is found that the low-lying optical phonon modes of Rb₂SnBr₆ are extremely soft and their phonon energies are almost strain independent, which ultimately lead to a lower κL and a weaker strain dependence than Cs₂SnBr₆. We further reveal that the strain dependence of these phonon modes in the A₂XB₆ -type perovskites weakens as their vibrational frequency decreases. This study deepens the understanding of lattice thermal transport in perovskites A₂XB₆ and provides a perspective on the selection of materials that meet the expected thermal behaviors in practical applications.