Thermoelectric Properties of Ga-Doped Ba8AlxSi46−x Clathrate

Single-crystalline Ba 8 Al x Ga y Si 46− x − y clathrates were synthesized by the arc melting method and Czochralski method without subsequent treatment, and their thermoelectric properties were compared with those of Ba 8 Al x Ga y Si 46− x − y and Ba 8 Al x Si 46− x clathrates with almost the same carrier concentration as estimated from the similar Seebeck coefficient and the Zintl concept. The resistivity of Ba 7.8 Al 5.3 Ga 7.4 Si 33.3 was lower than that of Ba 7.9 Al 12.6 Si 33.4 . The specific electrical resistance of Ba 7.9 Al 12.6 Si 33.4 and Ba 7.8 Al 5.3 Ga 7.4 Si 33.3 was 0.573 mΩ cm and 0.282 mΩ cm at 750 K, respectively. The band structure of Ba 8 Al 8 Ga 8 Si 30 and Ba 8 Al 16 Si 30 was estimated by first-principle calculations using density functional theory with the local density approximation. Based on these calculations, it was found that the shape of the bottom of the conduction band for Ba 8 Al x Si 46− x clathrate changed slightly on Ga doping and the radius of curvature of the bottom of the conduction band for Ba 8 Al 8 Ga 8 Si 30 clathrate was lower than that for Ba 8 Al 16 Si 30 clathrate. These results indicate that the mobility was enhanced by Ga doping of Ba 8 Al x Si 46− x clathrate. We also synthesized single-crystalline Ga-doped Ba 8 Al x Si 46− x clathrate. The electrical resistivity decreased dramatically due to the single-crystallization because of reduced electron scattering on grain boundaries. These results suggest that Ga doping and single-crystallization are effective for improvement of the thermoelectric properties of Ba 8 Al x Si 46− x clathrate.