Reduction of thermal conductivity and origin of carrier in alkaline-earth hexaborides

Mixed crystals of divalent hexaboride (CaB₆ and SrB₆) were prepared and their thermoelectric properties were measured. Seebeck coefficient and electrical conductivity of the mixed crystals were intermediate between those of CaB₆ and SrB₆ single phase. Thermal conductivities of the mixed crystals were lower than the both single phases. As a consequence, highest figure-of-merit was obtained at the vicinity of Ca_0.5Sr_0.5B₆, indicating that such a mixed crystallization using two or more hexaborides is effective to improve thermoelectric performance of these materials. For further improvement in their thermoelectric performance, the origin of charge carrier should be elucidated to control the electric properties. Rietveld analysis performed on X-ray diffraction data for CaB₆ revealed that there is a deficiency of boron in the sintered specimen. The boron deficiency appears to be one of possible origins of charge carrier.