Highly anisotropic crystal growth and thermoelectric properties of K ₂Bi _8-xSb _xSe ₁₃ solid solutions: Band gap anomaly at low x

The thermoelectric properties of solid solutions of the type β-K ₂Bi _8-xSb _xSe ₁₃ (0<x<8) were studied with respect to thermal behavior, band gap variation, and charge transport properties as a function of x. At x values between 0 and 1.5, the energy band gap is observed to decrease (anomalous) before it widens with increasing x values as would be expected. For selected members of the solid solutions, the Bridgman technique was applied to obtain well-grown oriented ingots that were used to measure the thermal conductivity and charge transport properties in different growth directions. The measurements showed a strong anisotropy in thermoelectric properties with the largest anisotropy observed in the electrical conductivity. Lattice thermal conductivities of the selected solid solutions were observed to decrease when the x value increases. Preliminary doping studies on the x=1.6 member were carried out and it was shown that it is possible to significantly increase the power factor.