High Thermoelectric Performance Achieved in Nb_0.8Ti_0.2FeSb via PbI₂-Driven Multiscale Defect Engineering

NbFeSb-based half-Heusler alloys offer high electrical conductivity and mechanical strength, yet suffer from high lattice thermal conductivity. Constructing complex microstructures to reduce thermal conductivity remains challenging due to high-temperature processing. This study introduces PbI₂ during ball-milling, which sublimates during sintering, creating a hierarchical structure in Nb_0.8Ti_0.2FeSb. The resulting features-PbI₂ nanophases, core-shell pore@Pb structures, multiscale porosity, and Fe vacancies-enable full-spectrum phonon scattering. Furthermore, the presence of Fe vacancies softens the lattice and reduces the sound velocity. Together, these reduce lattice thermal conductivity to 3.34 W m^-1 K^-1 at 973 K, a 32% decrease. Lowered grain-boundary barriers reduce hole trapping, increasing carrier concentration and electrical conductivity, leading to a power factor of 52.7 µW cm^-1 K^-2 and zT ~ 1. The material maintains high compressive strength (1132 MPa, 38% improvement) and microhardness (950 HV), as second-phase strengthening offsets pore-induced weakening. This approach demonstrates that sublimable compounds can form full-scale hierarchical architectures in high-temperature thermoelectrics, enabling both robust mechanical and thermoelectric performance.