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Charge ordering behavior is observed in the crystal prepared through the immersion of the N a0.41CoO2 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 N a0.41CoO2. The formal valence of Co changes from +3.59 estimated from the Na content to +3.50, the same as that in N a0.5CoO2. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayerhydrate. 1 The charge ordering is the same as that observed in N a0.5CoO2. It suggests that the Co valence of +3.50 is necessary for the charge ordering. * Corresponding author; Electronic address:

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