Valence states of copper ions and electronic structure ofLiCu2O2

The electronic structure of LiCu${}_{2}$O${}_{2}$ was studied using x-ray emission (Cu ${L}_{\ensuremath{\alpha}},$ O ${K}_{\ensuremath{\alpha}})$ and photoelectron spectroscopy (valence band and core levels) as well as band-structure calculations in terms of local spin-density approximation (LSDA) and LSDA+$U$ approaches. According to the x-ray-emission and photoelectron spectra the valence states of the Cu atoms are found to be mixed, i.e., 2+ and 1+. The LSDA calculations are contradictory to the experimental data and cannot reproduce the band gap and magnetic properties of ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$. The LSDA+$U$ calculations describe the insulator and antiferromagnetic properties much better but the overestimation of the screened Coulomb parameter $U$ leads to a binding-energy shift of the Cu${}^{\mathrm{II}}$ $3d$ states and this distorts the proper modeling of the valence-band structure. The magnetic structure of LiCu${}_{2}$O${}_{2}$ is discussed, taking our LSDA+$U$ band-structure calculations into account.