Nonstoichiometry and Transport Properties of Ca3Co4 ± x O9 + δ ( x = 0 – 0.4 )

Nonstoichiometries of Ca3Co4O9+δ and transport properties of Ca3Co4±xO9+δ were investigated. At 1100°C, Ca3Co4O9+δ transformed to CaO and CoO. The reaction products offer a precise baseline for thermogravimetric analysis. At room temperature, δ in Ca3Co4O9+δ is 0.38, which decreases at T ~450°C, indicating the onset point of the formation of oxygen vacancies, and δ is ~0.20 at 900°C. Correspondingly, the average Co valence state is 3.19 at room temperature and 3.10 at 900°C. In contrast to conventional defect chemistry theory in p-type oxide conductors, the formation of oxygen vacancies in Ca3Co4O9+δ 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 Ca/Co, the phase boundary for the misfit layered structure is between Ca3Co3.95O9+δ and Ca3Co4.05O9+δ. Beyond the phase boundary, the second phase is present, which effectively lowers the electrical conductivity while increasing the Seebeck coefficient.