Unusual oxygen re-equilibration kinetics of TiO2−δ

Oxygen re-equilibration kinetics, along with the equilibrium conductivity, have been measured on undoped, single-crystal TiO2−δ, by a four-probe d.c. conductivity relaxation technique, against oxygen partial pressure in the range of − 16 < log(PO2/atm) ≤ 0 at different temperatures in the range of 1173 ≤ T/K ≤ 1373. The isothermal conductivity varies as σ ∝ PO2m with m ≈ − 1/4, − 1/5 and − 1/6 in turn with increasing PO2 up to 1 atm, suggesting a sequential variation of the majority ionic disorder types from Tii to Tii to VO, respectively. Contrary to the conventional knowledge that due to the local (defect) equilibrium postulate there should be one and only one chemical diffusivity or single relaxation time for a binary oxide, the oxygen re-equilibration kinetics has turned out to be twofold with two different relaxation times depending on oxygen activities. This is interpreted as being due to the independent relaxation of each sublattice of TiO2 in an oxygen activity gradient applied, indicating a failure of local equilibrium during oxygen re-equilibration. From the two different relaxation times the chemical diffusion coefficients of component Ti and O are separately evaluated and subsequently, their self-diffusion coefficients. The latter are found to be in a good agreement with the literature data.