Local Orbital Occupation And Energy Levels Of Co In Naxcoo2: A Soft X-Ray Absorption Study

We present a combined experimental and theoretical investigation on the orbital occupation and local symmetry of Co in NaxCoO2. We have carried out polarization dependent Co-L-2,L-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)(+/-) and e(g)(+/-) orbitals. We have determined that the t(2g) holes in both the Na0.75CoO2 and Na0.5CoO2 compositions reside mainly in the a(1g) orbital. The spectroscopically deduced Co4+/Co3+ 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.