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 x Co y Sb Heusler alloys satisfying 2 x + 3 y = 3 exhibit semiconducting behavior with promising thermoelectric properties. Experimental results reveal that TiFe x Co y Sb 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 0.33 Sb achieves a low κ L of 2.77 W m −1 K −1 at room temperature. The optimal thermoelectric performance is achieved in TiFe 0.5 Co 0.67 Sb, 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.