Preparation and transport properties of non-hydrated Na₀.₅CoO₂ single crystals

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Single crystals of Na₀.₅CoO₂ were obtained through a flux method followed by de-intercalation of sodium. The Na₀.₅CoO₂ samples were found to be vulnerable to water in the air and a hydration process in which H₂O molecules fill oxygen vacancies in CoO₂ layers is suggested to be responsible for the unusual vulnerability to water. The transport properties, including resistivity (ρ), thermopower (S) and Hall coefficient (Rₕ), were studied in a temperature range of 5-300 K. The compound shows a weak localization of carriers just below 200 K and Co³⁺-Co⁴⁺ charge ordering at about 30 K, a relatively lower temperature than previously reported. The results seem to be quite different from those previously reported for this system [Foo et al, Phys. Rev. Lett. 92 (2004) 247001]. Possible mechanism underlying this kind of inconsistency is discussed.