Abstract
Single crystals of Na 0.495 CoO 2 were obtained through a flux method followed by de-intercalation of sodium. The resistivity (ρ) and thermal stability were studied for the de-intercalated samples exposed to humid air. It was found that the de-intercalated Na 0.495 CoO 2 crystal is vulnerable to water in the air, and a hydration process in which H 2 O molecules fill oxygen vacancies in CoO 2 layers instead of being intercalated between CoO 2 layers is suggested to be responsible for the remarkable change in resistivity induced by water absorption. The Na 0.495 CoO 2 sample shows a weak localization of carriers just below 200 K and Co 3+– Co 4+ charge ordering at about 40 K, lower than the corresponding charge ordering transition temperature reported in Na 0.5 CoO 2. The lower charge ordering transition temperature might be caused by the slight deviation of Na content (x) from 0.5.
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