Strong acoustic-optical phonon coupling in high-performance thermoelectric material Bi_0.5Sb_1.5Te₃

Understanding the origin of intrinsic lattice thermal conductivity in crystalline solids is essential for the optimization of thermoelectric properties. Here, we investigate the phonon dynamics of Bi_0.5Sb_1.5Te₃ by means of Raman scattering, specific heat measurement, and first-principles calculations. All the Raman modes display the three-phonon scattering processes below 490 K. The Boson peak of the low-temperature specific heat and the avoided-crossing of the phonon dispersion together reveal the existence of strong coupling between low-lying optical branches and acoustic branches. The vibration of Bi atoms is responsible for this acoustic-optical phonon coupling. Specifically, the lower group velocity, larger scattering space, larger scattering rate, and larger Grüneisen parameter near the avoided-crossing frequency lead to the lower lattice thermal conductivity of Bi_0.5Sb_1.5Te₃. This work provides new insights for understanding the thermal properties of Bi₂Te₃-based compounds.