Strong acoustic-optical phonon coupling in high-performance thermoelectric material Bi0.5Sb1.5Te3

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 Bi0.5Sb1.5Te3 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 Gruneisen parameter near the avoided-crossing frequency lead to the lower lattice thermal conductivity of Bi0.5Sb1.5Te3. This work provides new insights for understanding the thermal properties of Bi2Te3-based compounds.