Hard x-ray spectroscopy in NaxCoO2 and superconducting NaxCoO2 - yH2O: A view on the bulk Co electronic properties

The electronic properties of Co in bulk Na0.7CoO2 and the superconducting hydrated compound Na0.35CoO2 - y H2O have been investigated by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) using hard x-rays. The XAS spectra at the Co K-edge were measured in both compounds with two different polarization directions. The changes in the XAS spectra upon hydration and their polarization dependence are well accounted for by linear muffin- tin orbital calculations in the local density approximation. The underlying electronic structure indicates the strong hybridization between the Co 3d and O 2p states in both compounds, while the electron localization is enhanced in the hydrated compound due to the increase of the Co-Co interplanar distance. The Co K pre-edge further highlights the splitting of the d band as a result of the crystal field effect and demonstrates the Co valency increase when Na0.7CoO2 is hydrated. The RIXS spectra measured at the Co K-edge show an energy loss feature around 10 eV in both compounds in fair agreement with the calculated dynamical structure factor. The RIXS feature is associated to a damped plasmon excitation.