Oxidation state of cobalt in the NaxCoO2-delta center dot gamma H2O superconductor

Here we present results of accurate wet-chemical redox analyses revealing that the oxidation number of cobalt in the newly discovered CoO2-layer superconductor, NaxCoO(2)(.) yH(2)O (x approximate to 0.36, y approximate to 1.3) is similar to3.46. This value is significantly lower than the one (similar to3.64) 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 the 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.