SiGe alloys exhibit superior high-temperature thermoelectric performance and have consequently been implemented for decades in radioisotope thermoelectric generators (RTGs) of deep space probes. One of the reasons for their high thermoelectric conversion efficiency is the low thermal conductivity produced by phonon alloy scattering. In recent years, inelastic X-ray scattering experiments on SiGe crystals have revealed a new localized phonon mode in the low-energy range of 2–4 THz, which has been suggested to further reduce the thermal conductivity. Previous molecular-dynamics (MD) studies pointed out that this mode is related to collective vibrations of Si–Ge bonds surrounding Ge clusters; however, simulations of systems containing a wide distribution of cluster sizes could not reproduce the sharp spectral peak observed experimentally. In this work, we carried out MD simulations on systems with unified Ge cluster sizes, and a clear trend was observed that the intensity of the localized phonon mode increased as the cluster size decreased. Moreover, structures containing a high concentration of Ge dimers reproduced not only the sharp localized mode but also an intense Ge–Ge vibrational peak, in excellent agreement with the experimental spectrum. These results indicate that the SiGe alloys examined experimentally likely contained a high concentration of Ge dimers.
Miyagi et al. (Wed,) studied this question.