Impact of strain-insensitive low-frequency phonon modes on lattice thermal transport in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">A</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>X</mml:mi><mml:msub><mml:mi mathvariant="normal">B</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>-type perovskites

Abstract

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 ${\mathrm{Rb}}_{2}{\mathrm{SnBr}}_{6}$ and ${\mathrm{Cs}}_{2}{\mathrm{SnBr}}_{6}$ 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 (${\ensuremath{\kappa}}_{\mathrm{L}}$) calculations. It is found that the low-lying optical phonon modes of ${\mathrm{Rb}}_{2}{\mathrm{SnBr}}_{6}$ are extremely soft and their phonon energies are almost strain independent, which ultimately lead to a lower ${\ensuremath{\kappa}}_{\mathrm{L}}$ and a weaker strain dependence than ${\mathrm{Cs}}_{2}{\mathrm{SnBr}}_{6}$. We further reveal that the strain dependence of these phonon modes in the ${\mathrm{A}}_{2}X{B}_{6}$ -type perovskites weakens as their vibrational frequency decreases. This study deepens the understanding of lattice thermal transport in perovskites ${\mathrm{A}}_{2}{X\mathrm{B}}_{6}$ and provides a perspective on the selection of materials that meet the expected thermal behaviors in practical applications.

Topics & Concepts