Stabilization of 19-Electron Half-Heuslers: Vacancy Filling as a Pathway Toward Robust Thermoelectric Semiconductors

The simple electron counting rule is widely used to predict the phase stability and fundamental physical characteristics of Heusler alloys, while its effectiveness in facilitating the discovery of new Heusler compounds, particularly the critically needed Heusler semiconductors, is substantially limited. In this work, a vacancy-filling strategy based on the Zintl chemistry principles is developed to stabilize 19-electron half-Heusler alloys as stable thermoelectric semiconductors. Experimental results reveal that the partial filling of Fe into the 4d vacant sites of the unstable 19-electron half-Heusler alloys, such as TiNiSb, NbCoSb, and ZrNiBi, results in the formation of stable semiconductors TiNiFe_0.5Sb, NbCoFe_0.5Sb, and ZrNiFe_0.5Bi with bandgaps of 0.15, 0.16, and 0.14 eV, respectively. Notably, TiNiFe_0.5Sb achieves a thermoelectric figure of merit (zT) of 0.43 at 973 K, demonstrating its potential for thermoelectric applications. This study expands the design landscape of Heusler alloys and presents a promising pathway for the development of high-performance thermoelectric materials.