Unraveling electronic-structure features for metallic Na0.33CoO2 and   charge-ordered Na0.5CoO2 by high-resolution electron energy-loss spectroscopy
  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. 
R. J. Xiao, J. H. Chen, H. X. Yang, C. Ma, Q. Xu, H. W. Zandbergen,   and J. Q. Li