Here we present results of accurate wet-chemical redox analyses revealing that the oxidation number of cobalt in the newly discovered CoO2-layer superconductor, Na x CoO2·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.

The Na x CoO 2−␦ system-a host lattice for superb thermoelectrics and parent of the watery CoO 2 -layer superconductor-is found to be oxygen nonstoichiometric for x ഛ 0.7, such that with decreasing x, the concentration of oxygen vacancies increases. Accordingly, upon Na deintercalation, the Co valence first increases but does not exceed the value of 3.5. To reveal this, the actual valence of Co was systematically determined by means of wet-chemical redox analysis for a series of Na x CoO 2−␦ samples with the Na content x ranging from 0.78 down to 0.12. The low-Na-content samples were obtained from Na-rich ones ͑0.70ഛ x ഛ 0.78͒ through chimie douce extraction of Na utilizing I 2 , Br 2 , or NO 2 BF 4 as an oxidizing agent. For the lowest-Na-content sample of x = 0.12, the oxygen nonstoichiometry parameter ␦ was 0.23. The ␦-x relation presently established for a wide x range of Na x CoO 2−␦ is indispensable for proper interpretations of physical properties of this system.

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YMnO3 and the Fe doped samples were prepared and the lattice constants, electrical conductivities and Seebeck coefficients were measured. The solubility limit of Fe for Mn of YMnO3 was 25 atomic %, the electrical conductivity increased in the range of 350˚C-700˚C as Fe increased up to 10%, and the doped samples showed larger Seebeck coefficient than undoped samples.