Point defect structure of γ-NaxCoO2

Oxygen nonstoichiometry and electrical conductivity were measured on the system of γ-NaxCoO2, allegedly so far the best p-type thermoelectric oxide, against oxygen activity (aO2) across its widest ever range below aO2 = 1 for a fixed Na-content x = 0.706 at different temperatures in the range of 773-973 K, and at a fixed temperature 973 K for x = 0.664, 0.706 and 0.731, respectively. It has been deduced therefrom that as aO2 decreases, the majority disorder type shifts from [VNa′] ≈ p to either [VNa′] ≈ 2[CoNa⋅⋅] or [VNa′] ≈ 3[Coi⋯], however, exhibiting a positive deviation from the ideal defect behavior. The latter is attributed to the positive deviation of holes due to their degeneracy. By taking into appropriate account of the activity coefficient of holes in terms of the Fermi-Dirac integral of order 1/2, the nonstoichiometry and electrical conductivity have been precisely and consistently depicted to evaluate the defect-chemical parameters including the effective mass and mobility of holes and the redox equilibrium constant. The phase-stability limit of γ-NaxCoO2 is documented against temperature and Na-content.