Semiconducting Heusler Compounds beyond the Slater-Pauling Rule

Heusler compounds with semiconducting properties represent an important class of functional materials. Usually, research on these systems is guided by simple electron-counting rules, such as the Slater-Pauling principle. Here, we report on the discovery of Heusler-type semiconductors, significantly deviating from the Slater-Pauling rule. We theoretically predict the occurrence of nonmagnetic semiconducting ground states in various highly off-stoichiometric full-Heusler alloys, where self-substitution leads to a band-gap opening. This unexpected trend is confirmed experimentally by thermoelectric transport measurements on a multitude of $\mathrm{Fe}$${}_{2-2x}$$\mathrm{V}$${}_{1-x}$$\mathrm{Al}$${}_{1+3x}$ samples with up to 20% substitution of $\mathrm{Fe}$ and $\mathrm{V}$ atoms. The band-gap opening leads to an exceptionally large Seebeck coefficient in $p$-type $\mathrm{Fe}$${}_2$$\mathrm{VAl}$ thermoelectrics, previously limited by bipolar conduction and low-density-of-states effective mass. Consequently, our work presents a paradigm to tune the band gap of Heusler compounds by self-substitution and introduces a hitherto unexplored class of semiconductors with exceptional thermoelectric properties, offering significant potential for advancements in energy science and sustainable-energy technologies.