Insights into neutral vacancies and antisite defects and their defect engineering in TiIrSb half-Heusler alloy: first-principles electronic and optical effects

Abstract

Using first-principles density functional theory calculations, we systematically investigate the thermodynamic stability, defect energetics, and associated structural, electronic, magnetic, and optical properties of neutral native point defects in the half-Heusler (HH) alloy TiIrSb. The chemical-potential phase diagram establishes a well-defined single-phase stability domain constrained by seven thermodynamic vertices. Across the entire stability window, all calculated defect formation energies remain strictly positive (ΔEf >0), confirming the intrinsic thermodynamic stability of pristine TiIrSb against spontaneous defect formation. Among native vacancies, VIr is energetics-favored (ΔEf ≈1.33 eV under Ti-rich conditions), whereas the IrTi antisite exhibits the lowest overall formation energy (ΔEf ≈0.32 eV under Sb-rich conditions). Structurally, point defects induce localized atomic relaxations and minor directional strain within the 2 × 2 × 1 supercell, preserving the intrinsic HH atomic ordering within the tetragonal (a= b ≠ c) supercell geometry. Electronically, native defects introduce localized mid-gap states that effectively modulate the Fermi level position. Notably, magnetic polarization is restricted to specific configurations: while most defects remain non-magnetic, net magnetic moments and spin polarization are exclusively induced by VSb (0.9767 µB), TiIr (4.8771 µB), and SbIr (0.8322 µB). Optically, point defects substantially modify the dielectric response, generating pronounced low-energy absorption features in the infrared and visible spectra. These findings demonstrate that controlled defect engineering serves as an effective route to tailor the coupled electronic, magnetic, and optical response of TiIrSb without triggering secondary phase decomposition.