Electrocoloration in SrTiO3: Vacancy Drift and Oxidation-Reduction of Transition Metals

Application of dc electric fields, at temperatures of ∼100 to ∼325°C, to samples of SrTi${\mathrm{O}}_3$ doped with transition metals leads to the appearance of colored regions, characteristic of reduced material at the cathode and oxidized material at the anode. This process is reversible upon reversal of the polarity of the applied field. We have measured the space and time dependencies of the optical and electrical properties of such samples of SrTi${\mathrm{O}}_3$. All of the phenomena can be quantitatively understood in terms of a simple model, whose major ingredient is the drift of doubly positive oxygen vacancies across regions of high electric field, with accompanying oxidation and reduction of stationary transition-metal ions. Interpretation of our data yields quantitative estimates for the mobility of oxygen vacancies in this temperature range. For very long times of electrocoloration, the proposed model is inadequate for a quantitative description of the observed phenomena. This limitation is largely due to the formation of unusual oxidation states and complexes whose detailed nature is not yet completely understood.