Oxidation State of Cobalt in the NaxCoO2-δ·yH2O Superconductor

Here we present results of accurate wet-chemical redox analyses revealing that the oxidation number of cobalt in the newly discovered CoO 2 -layer superconductor, Na x CoO 2 · y H 2 O ( 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 Co IV lattice the phase should be considered as a hole-doped Co III lattice.