Local orbital occupation and energy levels of Co inNaxCoO2: A soft x-ray absorption study

Abstract

We present a combined experimental and theoretical investigation on the orbital occupation and local symmetry of Co in ${\text{Na}}_{x}{\text{CoO}}_{2}$. We have carried out polarization dependent $\text{Co}\ensuremath{-}{L}_{2,3}$ soft x-ray absorption spectroscopy measurements. By comparing the spectra with the results of configuration-interaction cluster calculations which include the full-atomic multiplet theory, we have found that the local symmetry for the Co ions are well-described by the ${D}_{3d}$ point group with intermixing between the cubic ${t}_{2g}^{\ifmmode\pm\else\textpm\fi{}}$ and ${e}_{g}^{\ifmmode\pm\else\textpm\fi{}}$ orbitals. We have determined that the ${t}_{2g}$ holes in both the ${\text{Na}}_{0.75}{\text{CoO}}_{2}$ and ${\text{Na}}_{0.5}{\text{CoO}}_{2}$ compositions reside mainly in the ${a}_{1g}$ orbital. The spectroscopically deduced ${\text{Co}}^{4+}/{\text{Co}}^{3+}$ ratio agrees well with the nominal Na concentration. We discuss the surprising observation that the single-site cluster approach is still adequate to reproduce the spectra despite the mixed valent character of the Co ions in this compound.