In Fe-doped SrTiO3 perovskite single crystals, the oxidation state of iron (substituting for Ti) depends sensitively on equilibration conditions (T, pO2). At increasing temperature or decreasing pO2, oxygen is gradually excorporated according to
OOx + 2 FeTix ↔ 1/2 O2 + VO.. + 2 FeTi' (1)
leading to the formation of oxygen vacancies VO.. and reduction of FeTix (corresponding to Fe4+) to FeTi' (Fe3+). Since Fe3+ in SrTiO3 is colorless while Fe4+ has a characteristic optical absorption at 590 nm, the kinetics of oxygen stoichiometry change can be monitored on a slice of Fe-doped SrTiO3 single crystal (larges faces sealed by glass) by in-situ optical spectroscopy. This allows us to analyze oxygen surface exchange kinetics, bulk chemical diffusion of oxygen and transport across grain boundaries (Fig. 1).