High Thermoelectric Performance Achieved in Nb _0.8 Ti _0.2 FeSb via PbI ₂ ‐Driven Multiscale Defect Engineering

ABSTRACT 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 PbI ₂ during ball‐milling, which sublimates during sintering, creating a hierarchical structure in Nb _0.8 Ti _0.2 FeSb. The resulting features—PbI ₂ 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.