Charge ordering in charge-compensated Na0.41CoO2 by oxonium ions

Charge ordering behavior is observed in the crystal prepared through the immersion of the Na0.41CoO2 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 Na0.41CoO2. The formal valence of Co changes from +3.59 estimated from the Na content to +3.50, the same as that in Na0.5CoO2. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate [K. Takada, et al. J. Mater. Chem. 14 (2004) 1448]. The charge ordering is the same as that observed in Na0.5CoO2. It suggests that the Co valence of +3.50 is necessary for the charge ordering.