Oxygen nonstoichiometry of Ca 3 Co 4 O y was precisely measured by thermogravimetry. Combining the result of thermal decomposition analysis, this compound was found to be expressed as (CoO 2 ) 1.62 (Ca 2 CoO 3-δ ) with δ=0–0.14. The observed oxygen nonstoichiometry in the present system was much larger than those of other layered cobaltites having the CdI 2 -type CoO 2 layer. Three regions showing different oxygen nonstoichiometry behaviors were found in the equilibrium oxygen phase diagram. Thermodynamic analyses revealed that dependencies of Δ h O 2 and Δ s O 2 on δ are apparently different in the three regions and the generation of oxygen defect accompanies a decrease of the valence of cobalt ions at the Ca 2 CoO 3-δ block in the main region.
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Shimoyama et al. (2003) studied this question.