Origin of the difference in thermal conductivity and anharmonic phonon scattering between LiNbO3 and LiTaO3

Abstract

The thermal transport behavior of single crystal LiNbO₃ and LiTaO₃ is crucially important in the application of laser devices; however, the underlying science of thermal conductivity of both crystals is still unknown. In the present study, we explore the origin of the different thermal conductivities between LiNbO₃ and LiTaO₃ based on first-principles calculation. We find that the remarkable contribution of optical phonon branches to the total thermal conductivity stems from the high phonon group velocity in the high frequency range (16–27 THz), and the anisotropic behavior of thermal conductivity can be regarded as the different phonon velocities along the c-axis and a-axis in the low frequency range (0–14 THz). As the decisive factor of thermal conductivity, the results of phonon lifetime indicate that the longer acoustic phonon relaxation times in LiTaO₃ are responsible for its higher thermal conductivity compared to LiNbO₃. The phonon–phonon scattering rates of three acoustic phonon branches reveal that more scattering channels exist throughout the Brillouin zone of LiNbO₃ than those of LiTaO₃. Additionally, the unusually large phonon scattering rate of the high-lying LA branch along the Γ–F direction indicates more emission channels of LiNbO₃, which is considered as another origin of the different phonon anharmonicities and thermal conductivities between LiNbO₃ and LiTaO₃.