Structural Understanding of the Slater-Pauling Electron Count in Defective Heusler Thermoelectric TiFe_1.5Sb as a Valence Balanced Semiconductor

Semiconducting properties in Heusler phases are of great interest for thermoelectric applications. Historically, transition metal based Heuslers semiconductors were associated with total valence electron counts (VECs) of 18 or 24 and were not expected to form at the compositions other than XYZ and XY₂Z. The semiconducting defective Heusler phase TiFe_1.5Sb─a low-cost example for emerging low thermal conductivity (κ) XY_1.5Z compounds─breaks both these stereotypes, stabilizing with an unusual VEC = 21. Although TiFe_1.5Sb is identified as a nonmagnetic semiconductor using the Slater-Pauling rule, this rule offers little structural understanding of its semiconducting properties. Using first-principles based electronic structure analysis, we establish that Fe─unlike in traditional semiconducting Heusler stoichiometries─acts both as a d⁶Fe²⁺cation and a covalently bonded d¹⁰Fe^2-species in TiFe_1.5Sb. In structures where these two types of Fe-atoms are indistinguishable by symmetry the electronic properties are metallic, indicating that a Slater-Pauling electron count alone does not guarantee semiconducting properties and thereby good thermoelectric efficiency. This insight into the semiconducting properties will assist in engineering thermoelectric performance of similar emerging low-κ compounds such as MRu_1.5+xSb and MCo_1.5Sn (M = Ti, Zr, and Hf).