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

TL;DR

This study uses x-ray spectroscopy techniques to analyze the electronic structure of cobalt in Na0.7CoO2 and its hydrated superconducting form, revealing hybridization, electron localization, and plasmon excitations.

Contribution

It provides detailed insights into the bulk Co electronic properties and their changes upon hydration, supported by experimental data and theoretical calculations.

Findings

Strong Co 3d and O 2p hybridization in both compounds

Enhanced electron localization in hydrated compound

Identification of a damped plasmon excitation around 10 eV

Abstract

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.