  Here we present results of accurate wet-chemical redox analyses, revealing
that the oxidation number of cobalt in the newly discovered CoO2-layer
superconductor, NaxCoO2 yH2O (x ~ 0.36, y ~ 1.3) is ~ 3.46. This value is
significantly lower than the one (~ 3.64) expected on the basis of the value
determined for the Na to Co stoichiometry ratio, x. The "lower-than-expected"
value for the Co oxidation state may be alternatively explained by presence of
oxygen vacancies or excess protons. Moreover, the result implies that rather
than an electron-doped Co(IV) lattice the phase should be considered as a
hole-doped Co(III) lattice.
