Al-doped SrTiO3: Part II, unusual thermodynamic factor and chemical diffusivity

For the system of SrTi1−xAlxO3−x/2−δ (x = 0.018), the thermodynamic factor (|∂ln aO2 / ∂δ|) of component oxygen was evaluated against oxygen activity in the range of 10− 18 ≤ aO2 ≤ 1, from the as-measured oxygen nonstoichiometry (δ) isotherms at elevated temperatures of 800° to 1000 °C (Part I). The factor would normally show a single maximum [at the close vicinity of the stoichiometric point (δ = 0)] in its near stoichiometry region (δ ≈ 0) or 2[VO
•
• ] ≈ [Al′Ti]. It has been found that the factor unusually exhibits two maxima due to the internal redox equilibrium of variable–valent background impurities or the like which are higher in concentration than intrinsic electrons at δ = 0. Oxygen chemical diffusivity was measured against aO2 by a conductivity relaxation technique in the same activity range at temperatures of 800° to 1100 °C. It appears to vary convex-upwardly up to the order of magnitude of 10− 3 cm2/s. This trend of variation as well as magnitude are satisfactorily explained in terms of the unusual thermodynamic factor and electronic transference number in accord with the classic Wagner theory of chemical diffusion.