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

Abstract

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 XY 2 Z. The semiconducting defective Heusler phase TiFe1.5Sba low-cost example for emerging low thermal conductivity (κ) XY 1.5 Z compoundsbreaks both these stereotypes, stabilizing with an unusual VEC = 21. Although TiFe1.5Sb 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 d6 Fe2+ cation and a covalently bonded d10 Fe2– species in TiFe1.5Sb. 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 MRu1.5+x Sb and MCo1.5Sn (M = Ti, Zr, and Hf).

Discovery of triple half-Heusler Mg2VNi3Sb3 with low thermal conductivity

“…6 The primary HH phase in these cases, actually accommodate large amounts of defects in the structure to become stable at the semiconducting NV = 0 compositions ( e.g. Nb 0.8 CoSb for NbCoSb and TiFe 1.5 Sb 55 in TiFeSb). Examples of such unstable metallic XYZ compositions containing impurity phases are NV = −1 based TiFeSb, 24 ZrRuSb, 25 TiRuSb, 26 HfRuSb 27 and NV = 1 based NbCoSb, 28 NbIrSb.…”

High Thermoelectric Performance and Low Lattice Thermal Conductivity in Lattice-Distorted High-Entropy Semiconductors AgMnSn1–xPbxSbTe4

“…Therefore, we performed DFT calculations on the SQS ,, of AgMnSnSbTe 4 and AgMnPbSbTe 4 , respectively (Figure ). The absence of the electronic states of the two spin channels at the Fermi level reflects their semiconductor nature.…”

Opening the Bandgap of Metallic Half‐Heuslers via the Introduction of d–d Orbital Interactions

“…Despite the VEC of TiNiSb not equaling 18, the forbidden d-d energy bandgap can still exist, but its electrical transport deviates from the intrinsic semiconductor behavior owing to the upshift of the Fermi level. [10] On the other hand, for the compounds with one transition metal, MgCoSb, and MgNiSb, the energy bandgap does not exist in their electronic band structures near the Fermi level (±1 eV). The projected DOS indicates that there is a negligible contribution of the Mg element in the MgYZ around the Fermi level, in contrast to the large contribution of the Ti element's d orbitals around the bandgap edge of TiYZ.…”