Abstract
The hexagonal CoO 2 layer is the main building unit of the newly established category of misfit-layered and related oxides showing—as a general feature—excellent thermoelectric characteristics. Here we use high-quality samples of the three prototype phases, Na 0.77CoO 2+ δ , Ca 3Co 3.95O 9+ δ and (Bi,Pb) 2Sr 2Co 2O 8+ δ , and apply both precise wet-chemical redox analysis techniques and thermogravimetric annealing experiments to approach the not-yet-addressed questions concerning oxygen nonstoichiometry and the valence of Co in these phases. In terms of the oxygen-stoichiometry tunability, substantial variation in the overall oxygen content upon reducing/oxidizing annealing is observed only for Ca 3Co 3.95O 9+ δ . The valence of Co in all the samples is found to lie in a range of 3.0–3.3, being significantly lower than the commonly believed values of 3.3–3.5.
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