The electronic structure of CuFeS₂, chalcopyrite, from x-ray emission and x-ray photoelectron spectroscopy and Xα calculations

The x-ray emission spectra (SK_β, SL, CuK_β, CuL, FeK_β, FeL) and x-ray photoelectron spectra of CuFeS ₂ chalcopyrite are reported and are interpreted using SCF-Xα MO calculations on the polyhedral anions CuS₄^-7 and FeS ₄^-5 and discrete variational Xα band structure calculations. The highest energy occupied orbitals are found to be of Cu3d-S3p antibonding character. Substantial Fe3d character is observed across a broad range of binding energies indicating strong mixing of the Fe3d orbitals with both the Cu3d and S3p. Both the Xα cluster calculation on FeS ₄^-5 and the Xα band structure calculations overestimate the Fe3d orbital binding energies, although the discrepancy is smaller in the band calculation. Both computational methods predict the lowest energy empty orbitals to be of Fe3d-S3p antibonding character with a small (<2 eV) separation from the highest occupied orbitals. This small energy difference provides a possible explanation for the metallic conductivity observed in CuFeS₂ at high pressure, particularly if the Cu ^T-S bond is more compressible than the Fe^III-S as is expected from compressibility-formal charge systematics.