Preparation and transport properties of non-hydrated Na${}_{0.5}$CoO${}_2$ single crystals

Abstract

Single crystals of Na${}_{0.5}$CoO${}_2$ were obtained through a flux method followed by de-intercalation of sodium. The Na${}_{0.5}$CoO${}_2$ samples were found to be vulnerable to water in the air and a hydration process in which H${}_2$O molecules fill oxygen vacancies in CoO${}_2$ layers is suggested to be responsible for the unusual vulnerability to water. The transport properties, including resistivity (\emph{$\rho$}), thermopower (\emph{S}) and Hall coefficient (\emph{R${}_H$}), were studied in a temperature range of 5-300 K. The compound shows a weak localization of carriers just below 200 K and Co${}^{3+}$-Co${}^{4+}$ 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.

9 pages, 5 figures