Thermopower and Co K-edge studies of potassium sodium cobalt oxyhydrates Na_(0.33)K_(0.02)(H_(2)O)_(1.33)CoO_(2-delta) and Na_(0.07)K_(0.21)(H_(2)O)_(0.63)CoO_(2-delta)

摘要

We report measurements of the thermopower and Co K-edge x-ray absorption spectroscopy onbilayer and monolayer (Na, K)_(x)(H_(2)O)_(y)CoO_(2-delta). According to the Co K-edge x-ray absorption spectra, the Co oxidation number for bilayer hydrate Na_(0.33)K_(0.02)(H_(2)O)_(1.33)CoO_(2-delta) is higher than that of monolayer hydrate Na_(0.07)K_(0.21)(H_(2)O)_(0.63)CoO_(2-delta) 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 Na_(x)(H_(2)O)_(y)CoO_(2-delta), 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 gamma-Na_(0.7)CoO_(2) is transformed to potassium sodium cobalt oxyhydrates.