  We have first used the aquesous KMnO4 solutions to de-intercalate and oxideze
gamma-phase of Na0.7CoO2 and successfully form the superconductive phase of
cobalt oxyhydrate hydrates (Na,K)xCoO2.yH2O with Tc=3.2-4.6K based on the
magnetization measurements. The higher molar ratio of KMnO4/Na used to treat
Na0.7CoO2 results in more removal of Na+ and leads to a partial or even
complete replacement of K+ for Na+. The low molar ratio of KMnO4/Na forms a
superconductive phase with the c-axis ca. 19.6 angstrom, whereas the high molar
ratio of KMnO4/Na forms a non-superconductive phase with teh c-axis ca. 13.9
angstrom. The superconductive 19.6 angstrom phase is unstable with respect to
the ambient air in terms of losing water molecule from the structure;
nevertheless, the dehydration/hydration process is reversible when storing the
sample in a chamber with sufficient humidity.

Superconductivity;Materials Science