Contrasting role of bismuth doping on the thermoelectric performance of VFeSb half-Heusler

Isoelectronic heavy elemental substitutions are generally preferred for enhancing the phonon scattering in a material without deteriorating its electrical transport. In this work, we demonstrate the efficacy and contrasting role of bismuth, an isoelectronic substitute of antimony, for enhancing the thermoelectric transport in structurally ordered cubic VFeSb half-Heusler. Despite its limited solubility, Bi-substitution at Sb-site, was found to be effective in enhancing the electrical power factor near room temperature. Alongside, synergistic lowering of lattice thermal conductivity was observed due to point defect scattering of phonons by Bi-induced mass and strain fluctuations. The electronic transport properties of nominal compositions and effects of Bi-doping induced disorder were evaluated using the Korringa–Kohn–Rostoker method with a coherent potential approximation (KKR-CPA). A highest thermoelectric figure-of-merit (zT) ~0.22 at 478 K was attained with improved weighted mobility and quality factor for optimally doped VFeSb0.98Bi0.02 half-Heusler alloy making them prospective compositions for near room temperature thermoelectric applications. •Contrasting role of bismuth as prospective substitute for isovalent antimony in Sb-based half-Heusler.•High-power factor > 3.0 × 10−3 Wm−1 K−2 at room temperature.•Lattice thermal conductivity reduction by point defect scattering in Bi-substituted VFeSb alloys.•Enhanced weighted mobility, thermoelectric quality factor and zT in Bi-substituted VFeSb alloys.