Oxidation State of Cobalt in the NaxCoO2-·yH2O 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, 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 the presence of oxygen vacancies or excess protons. Moreover, the result implies that rather than an electron-doped CoIV lattice the phase should be considered as a hole-doped CoIII lattice.