Electronic structures of CuFeS2 and CuAl0.9Fe0.1S2 studied by electron and optical spectroscopies

The electronic structures of the chalcopyrite-type CuFeS2 and CuAl0.9Fe0.1S2 are studied by x-ray photoemission (XPS), resonance photoemission, Auger-electron, optical reflectance, and electron-energy-loss (EELS) spectroscopies. The Fe 3d-derived states are revealed by the valence-band XPS spectra and the Fe 3p core resonance photoemission spectra. The spectra are analyzed by configuration-interaction calculation on the FeS4 cluster model; the analysis yields the S 3p→Fe 3d charge-transfer energy Δ close to zero, indicating strong covalency between the Fe 3d and S 3p orbitals. This situation is reflected upon the reduced Fe magnetic moment and the high Neel temperature of CuFeS2. The S 3p→Fe 3d charge-transfer excitation is resolved in the optical reflectance and EELS spectra, which explains the larger binding-energy tails of the core-level photoemission spectra of CuFeS2. The Cu 3d two-hole bound state is studied through the Cu L3M4,5M4,5 Auger and Cu 3p core resonance photoemission spectra, from which the effective Coulomb energy Ueff(1G) between the two holes and the Cu 3d→4sp promotion energy Δd-sp are evaluated. The Cu 2p core XPS spectrum of CuFeS2 has revealed a mixing of the d9 (''Cu2+'') configuration into the formally monovalent Cu. This is interpreted as due to the Cu 3d-Fe 3d hybridization mediated by the S 3sp valence states.