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

Abstract

The electronic structures of the chalcopyrite-type ${\mathrm{CuFeS}}_2$ and ${\mathrm{CuAl}}_{0.9}$ ${\mathrm{Fe}}_{0.1}$ ${\mathrm{S}}_2$ 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 ${\mathrm{FeS}}_4$ 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 ${\mathrm{CuFeS}}_2$. 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 ${\mathrm{CuFeS}}_2$. The Cu 3d two-hole bound state is studied through the Cu ${\mathit{L}}_3$ ${\mathit{M}}_{4,5}$ ${\mathit{M}}_{4,5}$ Auger and Cu 3p core resonance photoemission spectra, from which the effective Coulomb energy ${\mathit{U}}_{\mathrm{eff}}$ ${(}^1$G) between the two holes and the Cu 3d→4sp promotion energy ${\mathrm{\Delta }}_{\mathit{d}\mathrm{-}\mathit{s}\mathit{p}}$ are evaluated. The Cu 2p core XPS spectrum of ${\mathrm{CuFeS}}_2$ has revealed a mixing of the ${\mathit{d}}^9$ (‘‘${\mathrm{Cu}}^{2+}$’’) 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.