  The impact of oxygen deficiency on physical properties of Na2/3CoO2-x has
been investigated. From the combined thermogravimetric, magnetic susceptibility
and synchrotron X-ray Laue diffraction studies, it is demonstrated that
Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98,
which has the same Co valence as that of Na0.71CoO2, shows nearly identical
magnetic and transport properties and the same simple hexagonal superlattice
ordering of sqrt(12)a. It is proposed that the Na ion ordering found in
Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large sqrt(12)a x
sqrt(12)a x 3c superlattice but with additional Na vacancies which are bound to
the oxygen defects at room temperature. We conclude that oxygen vacancies play
a key role in stabilizing the superlattice structure and must be accounted for
in its modeling.
