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Clathrate compounds are promising thermoelectric materials that embody the phonon-glass electron-crystal concept. Among the various types of clathrate compounds, those based on silicon are particularly favored for practical applications owing to their low cost. In this study, we synthesize type-I clathrate compounds Ba 8 Ga 16– x–y Al x Si 30+ y ( x = 5.5 to 7.0 and y = −3.0 to −1.5). By modulating the carrier concentration via optimization of the initial composition and sintering temperature, we achieve record-breaking values of the dimensionless thermoelectric figure of merit ( ZT ) for silicon-based clathrates: ZT = 0.72 at 873 K. Additionally, we conduct a detailed analysis of their thermoelectric properties using the first-principles calculations for crystal structures while considering the thermodynamic stability at a given effective process temperature, employing cluster expansion and Monte Carlo approaches. These calculations reproduce the experimentally measured properties significantly better than those obtained by the usual ground-state calculations. The analysis suggests that the carrier concentration obtained in this study is nearly optimal, explaining the improved ZT, and that further moderate improvement is possible by reducing the process temperature. This study provides a clear and practical pathway to enhance the performance of silicon-based clathrates, promoting their use in a variety of applications where efficient thermal-to-electric energy conversion is desirable.
Ohnishi et al. (Tue,) studied this question.