Strain-driven anisotropic enhancement in the thermal conductivity of KCaBi: the role of optical phonons

Acoustic phonons have long been believed to dominate the lattice thermal conductivity (${\kappa }_l$) and the contribution of optical phonons can be neglected in crystal structures. KCaBi, as a high-throughput screening semiconductor with ultralow ${\kappa }_l$ [J. Am. Chem. Soc. 144, 4448 (2022)], has been demonstrated that the contribution of optical phonons plays an important role in thermal transport. In this work, by solving the Boltzmann transport equation, it is found that the ${\kappa }_l$ of KCaBi is 2.2 at 300K, with acoustic phonons dominating the z-direction ${\kappa }_l$ and optical phonons contributing around 50% to the x-direction ${\kappa }_l$ under the four-phonon picture. The uncommon contribution of optical phonons also manifests the possibility of tuning the ${\kappa }_l$ anisotropy based on optical phonons. Following this line of thinking, it is found that applying tensile strain can cause a more pronounced decrease of acoustic phonon contribution than that of optical counterpart due to the highly dispersive optical branches, thus enhancing the anisotropic ratio of ${\kappa }_l$. Moreover, the microscopic mechanism is elucidated by analyzing the phonon dispersion relation, phonon mode-wise contribution and phonon scattering rates. Our study could provide appealing alternatives for the regulation of phonon transport from the viewpoint of optical phonons.