Charge ordering in charge-compensated $Na_{0.41}CoO_2$ by oxonium ions
  Charge ordering behavior is observed in the crystal prepared through the immersion of the $Na_{0.41}CoO_2$ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the   $Na_{0.41}CoO_2$. The formal valence of Co changes from +3.59 estimated from the   Na content to +3.5, the same as that in $Na_{0.5}CoO_2$. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate.\cite{takada1}   The charge ordering is the same as that observed in $Na_{0.5}CoO_2$. It suggests that the Co valence of +3.5 is necessary for the charge ordering. 
C. H. Wang, H. T. Zhang, X. X. Lu and G. Wu, J. Q. Li, X. H. Chen