Nonideal defect structure and high-temperature transport properties of misfit-layered cobalt oxide

In the study, the oxygen nonstoichiometry (δ), electrical conductivity (σ), and thermal expansion of misfit cobalt oxide (Ca3Co4O9+δ) were measured at various temperatures (RT ​≤ ​T/°C ​≤ ​850) and oxygen partial pressures (3.16 ​× ​10−5 ​≤ ​Po2/atm ≤ 10°). The dense specimens used in the experiment were produced through spark plasma sintering. Generally, the defect structure of the nonstoichiometric compound exhibits a negative deviation as defect concentration increases owing to the Coulomb attraction of oppositely charged point defects. However, δ and σ deviated positively from the ideal defect structure. For a positive deviation, the nonideal defect structure and the degenerated holes could accurately explain the physical properties. The activity coefficient of holes from the Fermi-Dirac integration using the Joyce–Dixon approximation increased from 2.5 to 11 as δ increased from 0.09 to 0.175. The anisotropic nature was confirmed and the thermal expansion showed orientation dependence (in-plane: 12 ppmK-1, out-of-plane: 13.1 ppmK-1). Chemically induced expansion was not observed regardless of the value of δ or the orientation.