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

The thermal transport behavior of single crystal LiNbO3 and LiTaO3 is crucially important in the application of laser devices, however, the underlying sciences of thermal conductivity of both crystals are still unknown. In present study, we explore the origin of different thermal conductivity between LiNbO3 and LiTaO3 based on first-principle calculation. We find that the remarkable contribution of optical phonon branches to the total thermal conductivity stems from the high phonon group velocity at high frequency range (500-900 cm-1), and the anisotropic behavior of thermal conductivity can be regarded as the different value of phonon velocity along c- and a-axis at the low frequency range (0-400 cm-1). As the decisive factor in thermal conductivity, the results of phonon lifetime indicate that large value of acoustic phonon relaxation time in LiTaO3 are responsible for the higher thermal conductivity compared to LiNbO3. The phonon-phonon scattering rates of three acoustic phonon branches reveal that more scattering channels are existed throughout the Brillouin zone for LiNbO3 than that of LiTaO3. Additionally, the unusually large phonon scattering rate of 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 anharmonicity and thermal conductivity between LiNbO3 and LiTaO3.