An Alternative Approach to Improve the Thermoelectric Properties of Half-Heusler Compounds

In this report an alternative approach for optimization of the thermoelectric properties of half-Heusler compounds is presented. The common approaches are partial substitution of elements by elements of nearby groups and substitution with homologs. In this approach we substitute one element by one neighboring element with fewer valence electrons and by one with more electrons. The amounts of the substitutions are chosen such that the amount of deficiency and excess electrons are compensated. In the solid solution TiCox(Ni0.5Fe0.5)(1-x)Sb, Co was substituted equally by Fe and Ni. The aim of the substitution was to improve the figure of merit by a reduction of the thermal conductivity accompanied by an unchanged high Seebeck coefficient. The solid solution TiCox(Ni0.5Fe0.5)(1-x)Sb, Co was synthesized by arc-melting. The structure of the as-cast samples was analyzed by x-ray diffraction. Rietveld refinements yielded the TiCox(Ni0.5Fe0.5)(1-x)Sb, Co structure type with a small amount of antisite disorder between Co and Sb. The thermoelectric properties of the solid solution were investigated in the temperature range from 2 K to 400 K. A Seebeck coefficient of TiCox(Ni0.5Fe0.5)(1-x)Sb, Co at 400 K and a reduction of the thermal conductivity to TiCox(Ni0.5Fe0.5)(1-x)Sb, Co were measured. The figure of merit was enhanced by a factor of about seven to a value of 0.04 at 400 K for TiCox(Ni0.5Fe0.5)(1-x)Sb, Co.