Nonstoichiometry and Transport Properties of Ca[sub 3]Co[sub 4±x]O[sub 9+δ] (x=0–0.4)

Nonstoichiometries of and transport properties of were investigated. At , transformed to CaO and CoO. The reaction products offer a precise baseline for thermogravimetric analysis. At room temperature, δ in is 0.38, which decreases at , indicating the onset point of the formation of oxygen vacancies, and δ is at . Correspondingly, the average Co valence state is 3.19 at room temperature and 3.10 at . In contrast to conventional defect chemistry theory in p-type oxide conductors, the formation of oxygen vacancies in has a negligible impact on the carrier density of holes, indicating that oxygen vacancies and the redox couple responsible for hole carriers are in different layers. With control over the ratio of , the phase boundary for the misfit layered structure is between and . Beyond the phase boundary, the second phase is present, which effectively lowers the electrical conductivity while increasing the Seebeck coefficient.