Unraveling electronic-structure features for metallic Na0.33CoO2 and charge-ordered Na0.5CoO2 by high-resolution electron energy-loss spectroscopy

TL;DR

This study uses high-resolution electron energy-loss spectroscopy and DFT+U calculations to compare the electronic structures of metallic Na0.33CoO2 and charge-ordered Na0.5CoO2, revealing how electron correlation influences their properties.

Contribution

It provides new insights into the electronic structure differences driven by electron correlation and crystal-field effects in Na_xCoO_2 compounds.

Findings

Spectral features differ significantly between Na0.33CoO2 and Na0.5CoO2.

Estimated U values are ~1.0 eV for x=0.33 and ~3.0 eV for x=0.5.

Electronic structure interpretations align with observed physical properties.

Abstract

Measurements by high-resolution electron energy-loss spectroscopy (HREELS) of NaxCoO2 reveal spectral features that differ remarkably between the metallic Na0.33CoO2 and the charge-ordered insulator Na0.5CoO2. Calculations by density functional theory plus Hubbard U (DFT+U) demonstrate that these differences arise essentially from the relatively greater strength of electron correlation in addition to the crystal-field effect in Na0.5CoO2. The effective U values are estimated to be ~3.0eV for x=0.5 and ~1.0eV for x=0.33, respectively. The electronic structures for these correlation strengths give good interpretations for the physical properties observed in the materials.