A mechanistic view of defect engineered VFeSb half-Heusler alloys

Abstract

Defects are ubiquitous and extensively found in a half-Heusler microstructure, and often yield a significant variation in its transport properties. There remains a critical gap in understanding the nature and origin of such prevalent defects that influence the electronic band structure and lattice dynamics of the innately disordered half-Heusler alloys. Here, we employ thermodynamics evaluation and structural characterization to understand the microscopic origin of a defect sub-structure and elucidate the critical role of stoichiometry by examining defective V_1+xFe_1+ySb (−0.1 < x, y < 0.1) half-Heusler compositions. It was found that microstructural metastability and an inherent tendency for atomic ordering result in vacancies/interstitials, which can be enhanced in off-stoichiometric compositions. These atomic disorders are embodied as V-segregates, Fe-rich dendrites, and V-deficient eutectic substructures within the structurally ordered half-Heusler microstructure while their concentration depends on the altered stoichiometry. A significant variation in both power factors and lattice thermal conductivity was observed, which is more favorable for V-excess and Fe-deficient compositions in low self-doping limits (0 < x < 0.05) for thermoelectric applications. These findings highlight the defective nature of cubic n-type VFeSb half-Heusler alloys and elucidate the implication of prevalent defects on thermoelectric transport properties.