Thermopower and Co K -edge studies of potassium sodium cobalt oxyhydrates Na0.33 K0.02 (H2 O)1.33 Co O2-δ and Na0.07 K0.21 (H2 O)0.63 Co O2-δ

We report measurements of the thermopower and Co K -edge x-ray absorption spectroscopy on bilayer and monolayer (Na,K)x (H2 O)y Co O2-δ. According to the Co K -edge x-ray absorption spectra, the Co oxidation number for bilayer hydrate Na0.33 K0.02 (H2 O)1.33 Co O2-δ is higher than that of monolayer hydrate Na0.07 K0.21 (H2 O)0.63 Co O2-δ with a chemical shift of 3.5 eV, which is in sharp contrast with Co K -edge x-ray absorption spectroscopy and cerimetric titration results for Nax (H2 O)y Co O2-δ, where there is essentially no difference of the oxidation number of Co between bilayer and monolayer sodium cobalt oxyhydrates. According to thermopower analysis, band narrowing is observed when γ- Na0.7 Co O2 is transformed to potassium sodium cobalt oxyhydrates.