Photochromism of three photosensitive Fe centers in SrTiO3

Three paramagnetic Fe defect complexes have been observed by electron-spin resonance in chemically reduced crystals of SrTi${\mathrm{O}}_3$. These are identified as (1) an ${\mathrm{Fe}}^{3+}$ ion substituted on a Ti site with a nearby charge-compensating interstitial impurity, and (2) and (3) two charge states of substitutional Fe with a near-neighbor oxygen vacancy. ${\mathrm{Fe}}^{2+}$-$V_0$ and ${\mathrm{Fe}}^{1+}$-$V_0$. These three defects are related in the sense that photoexcitation can reversibly switch electrons between the vacancy-associated and interstitial-associated sites. An analysis of the response to polarized light, and of the kinetics of charge transfer, shows that in both the forward and reverse directions of optical pumping the mobile charge carrier is a valence-band hole. Four distinct charge states of the Fe-$V_0$ system have now been identified, and the relative instability of SrTi${\mathrm{O}}_3$ toward formation of multiple-impurity charge states is briefly discussed.