Thermoelectric properties and structural instability of type-I clathrate Ba ₈Ga ₁₆Sn ₃₀ at high temperatures

Type-I clathrate Ba ₈Ga ₁₆Sn ₃₀ is known as a typical example showing glass-like behavior in the thermal conductivity at low temperatures. We report on thermoelectric properties above room temperature for the p- and n-type single crystals which were grown from Ga-Sn double flux and Sn single flux, respectively. The measurements of electrical resistivity showed hysteretic behaviors when the sample was heated to 600 K. Powder X-ray diffraction analysis indicated that the type-I structure changed to the type-VIII after the sample was heated to 600 K. By using the data of Seebeck coefficient, electrical resistivity, and thermal conductivity, we estimated the dimensionless figure of merit ZT for the type-I Ba ₈Ga ₁₆Sn ₃₀. For the p- and n-type samples, the values of ZT reach 0.58 and 0.50 at around 450 K, respectively, which values are approximately half of those for the type-VIII counterparts.