Design Principles and Experimental Evidence for Semiconducting Heusler Thermoelectrics by Vacancy-Filling

Our previous experimental studies have established the vacancy-filling strategy as an effective approach for expanding the Heusler family. In this work, we propose that vacancy-filling Heusler semiconductors can be designed by creating an energy gap (i.e., bandgap) between the fully occupied t_2g and empty e_g orbitals. Our experiments confirm that non-stoichiometric TiFe_xCo_ySb Heusler alloys satisfying 2x + 3y = 3 exhibit semiconducting behavior with promising thermoelectric properties. Experimental results reveal that TiFe_xCo_ySb samples show a p-type conduction, and their lattice thermal conductivity (κ_L) is significantly reduced due to partial occupation of the 4d crystallographic sites. Remarkably, TiFeCo_0.33Sb achieves a low κ_L of 2.77 W m^-1 K^-1 at room temperature. The optimal thermoelectric performance is achieved in TiFe_0.5Co_0.67Sb, which attains a dimensionless thermoelectric figure of merit of 0.53 at 973 K. These findings highlight the potential of this strategy to design semiconducting Heuslers with tunable and enhanced thermoelectric properties.