Hard x-ray spectroscopy inNaxCoO2and superconductingNaxCoO2∙yH2O: Bulk Co electronic properties

Abstract

The electronic properties of Co in bulk ${\mathrm{Na}}_{0.7}\mathrm{Co}{\mathrm{O}}_{2}$ and the superconducting hydrated compound ${\mathrm{Na}}_{0.35}\mathrm{Co}{\mathrm{O}}_{2}∙y{\mathrm{H}}_{2}\mathrm{O}$ 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 $K$ preedge further highlights the splitting of the $d$ band as a result of the crystal field effect and demonstrates the Co valency increase when ${\mathrm{Na}}_{0.7}\mathrm{Co}{\mathrm{O}}_{2}$ is hydrated. The RIXS spectra measured at the Co $K$ edge show an energy loss feature around $10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in both compounds, in fair agreement with the calculated dynamical structure factor. The RIXS feature is associated to a damped plasmon excitation.