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

Cu-deficient bornite Cu_5-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 Cu_4.80FeS₄. 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, Cu_4.80FeS₄ 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.