Charge Transport and Thermoelectric Performance of Bornite Controlled by Cu Non-Stoichiometry

Cu-deficient bornite Cu5−xFeS4 (x = 0–0.20) samples were synthesized by mechanical alloying (MA) followed by hot pressing (HP) to investigate the effects of Cu-site non-stoichiometry on structural evolution, charge transport, and thermoelectric performance. X-ray diffraction confirmed single-phase bornite formation after MA, whereas a minor chalcopyrite CuFeS2 secondary phase appeared in highly Cu-deficient samples after HP, indicating reduced phase stability during thermal consolidation. Rietveld refinement revealed anisotropic lattice distortion and a gradual decrease in unit-cell volume with increasing Cu deficiency. Hall-effect measurements showed that Cu vacancies act as acceptor defects, increasing the hole concentration to the order of 1018–1019 cm−3 while reducing carrier mobility through enhanced defect scattering. Consequently, the electrical conductivity increased, whereas the Seebeck coefficient decreased with increasing Cu deficiency. The power factor was enhanced, reaching 0.46 mW m−1 K−2 at 723 K for Cu4.80FeS4. The thermal conductivity remained low at 0.48–0.77 W m−1 K−1 owing to dominant lattice contributions and intensified phonon scattering. As a result, Cu4.80FeS4 exhibited a 52% higher ZT at 523 K than stoichiometric Cu5FeS4, demonstrating that Cu deficiency is an effective strategy for tuning carrier concentration and defect structure in bornite.