Thermal Stability And Transport Properties Of Na0.495coo2 Single Crystals

Single crystals of Na0.495CoO2 were obtained through a flux method followed by deintercalation of sodium. The resistivity (p) and thermal stability were studied for the de-intercalated samples exposed to humid air. It was found that the de-intercalated Na0.495CoO2 crystal is vulnerable to water in the air, and a hydration process in which H2O molecules fill oxygen vacancies in CoO2 layers instead of being intercalated between CoO2 layers is suggested to be responsible for the remarkable change in resistivity induced by water absorption. The Na0.495CoO2 sample shows a weak localization of carriers just below 200 K and Co3+-Co4+ charge ordering at about 40 K, lower than the corresponding charge ordering transition temperature reported in Na0.5CoO2. The lower charge ordering transition temperature might be caused by the slight deviation of Na content (x) from 0.5.