Hard x-ray spectroscopy in Na x Co O 2 and superconducting Na x Co O 2 ∙ y H 2 O : Bulk Co electronic properties
Résumé
The electronic properties of Co in bulk Na 0.7 CoO 2 and the superconducting hydrated compound Na 0.35 CoO 2 · yH 2 O have been investigated by x-ray absorption spectroscopy ͑XAS͒ and resonant inelastic x-ray scattering ͑RIXS͒ 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 preedge further highlights the splitting of the d band as a result of the crystal field effect and demonstrates the Co valency increase when Na 0.7 CoO 2 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.
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