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

TL;DR

This study investigates the synthesis and transport properties of non-hydrated Na$_{0.5}$CoO$_2$ single crystals, revealing unique water sensitivity, charge ordering at lower temperatures, and differences from prior reports.

Contribution

It provides new insights into the transport behavior and water vulnerability of Na$_{0.5}$CoO$_2$, highlighting differences from previous studies.

Findings

Na$_{0.5}$CoO$_2$ crystals are water-sensitive due to oxygen vacancies.

Charge ordering occurs at about 30 K, lower than previously reported.

Transport measurements show weak localization below 200 K.

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.