We reported the thermoelectric properties of the sintered type-II clathrate K8Ba16Ga40Sn96 in a previous paper [S. Koda et al., J. Appl. Phys. 116, 023710 (2014)]. The clathrate had a high dimensionless figure of merit ZT, namely, 0.93. In this study, we optimized the carrier concentration n by modifying the chemical compositions of (K, Ba)24(Ga, Sn)136 samples, and heat treated the sintered samples. The carrier mobilities μ were improved because of the reduction in potential barrier scattering at grain boundaries. The room-temperature (RT) n values varied from 7.7 × 1017 to 3.7 × 1019 cm−3; the maximum RT μ value was 170 cm2V−1s−1. Consequently, we obtained a high ZT value of 1.19 at 630 K for n = 2.5 × 1019 cm−3. This material therefore has good thermoelectric properties.

We calculated the electronic structures and the thermoelectric properties for type-I and type-VIII Ba 8 Ga 16 Sn 30 ͑BGS͒ clathrates. The band structures show that type-I and type-VIII BGS are indirect semiconductors with band gaps of 0.51 eV and 0.32 eV, respectively. The calculated Seebeck coefficient of n-type type-I BGS is higher than that of n-type type-VIII BGS because of the larger density of states in type-I at the bottom of the conduction band. This is in good agreement with the experimental results. We also calculated the electrical conductivity and thermal conductivity due to charge carriers. Estimated thermoelectric figure of merit, ZT, exceeds 1.0 for both types.

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