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.Comment: 8 page