Oxygen Surface Exchange on Stepped (100) Surfaces of Strontium Titanate

The influence of surface steps on the rate of oxygen surface exchange was investigated for a perovskite-type oxide by means of 18O2/16O2 exchange. Seven vicinal (100)-oriented single-crystal samples of acceptor-doped SrTiO3 with TiO2-terminated surfaces and miscut angles between 0 and 3° were used, giving a variation in step density (ρst) of a factor of approximately 7. All samples were subjected together to a single high-temperature isotope exchange anneal (at T = 1023 K in pO2 = 0.1 bar, with pH2O < 10–7 bar) and were then analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Numerical descriptions of the isotope diffusion profiles yielded the oxygen tracer diffusion coefficient for the bulk (D*), the oxygen isotope exchange coefficient for the surface (k*), and the space-charge potential at the surface (Φ0). Surprisingly, the k* data exhibited no increase with increasing ρst, suggesting that surface steps do not strongly promote oxygen surface exchange in this system under these conditions. The behavior of Φ0 as a function of ρst is consistent with preferential formation of oxygen vacancies both at under-coordinated oxide-ion sites of terraces and at under-coordinated oxide-ion sites of step edges and terraces.