X-ray photoelectron spectroscopy of Cu³⁺ in NaK₂CuF₆

Abstract

The valence-band, Cu 3p and Cu 2p core-level spectra of NaK₂CuF₆, cryolite, containing copper in a 3 + valence state have been measured and the results are compared with those of other Cu³⁺ compounds such as LaCuO₃ and NaCuO₂. Evidence of the 2p⁵3d⁸ final state in Cu 2p core-level and 3d⁷ satellites in the valence band due to Cu³⁺ ions has been found. The L₃M₄₅M₄₅(LVV) Auger-electron kinetic energy is shifted by 1.2 eV relative to that of CuF₂. The satellite of the LVV Auger line and the 2p and 3p core-level spectra are explained in terms of final-state energies diminished by Coulomb interactions between the valence electrons and the X-ray induced 2p core holes. The fluorine is binding energy (686.3 eV) indicates more ionic character and less charge transfer from ligand to metal. Experimental results are explained with simple configuration interaction calculations and the nature of the cryolite as a charge-transfer insulator.