Preparation and transport properties of non-hydrated Na$_{0.5}$CoO$_2$ single crystals
msra(2004)
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.
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Key words
single crystal,weak localization,materials science
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