Anomalous Temperature-Dependent Phonon Anharmonicity and Strain Engineering of Thermal Conductivity in β-Ga2O3

β-Ga2O3 is a promising candidate for high-performance devices such as high-power electronics, but the low lattice thermal conductivity κ seriously hinders its application. In this paper, by comparing the κ at 200–700 K calculated by the temperature-dependent and fixed second- and third-order force constants, the softening of optical phonons and the weakening of anharmonicity with increasing temperature are revealed. The lattice thermal conductivity along the crystal orientation [010] ([100] and [001]) is significantly increased from 23.74 W/mK (10.02 and 12.00 W/mK) to 35.58 W/mK (16.76 and 18.38 W/mK) due to the application of 4% compressive uniaxial strain along the y(z) direction. The improvement of thermal transport properties is attributed to the increase in heat capacity, phonon group velocity, and relaxation time caused by the decrease in volume, strengthening of polar bonds, and decrease in three-phonon scattering channels, respectively. It is worth noting that the compression along different directions causes a change in different bond angles, which leads to the improvement or reduction of crystal symmetry and further leads to anisotropic changes in anharmonicity. Our results pave the way for further mechanism research and strain engineering of thermal transport properties of β-Ga2O3 and guiding novel design for the application of β-Ga2O3-based materials.