Photoinduced Reversible Charge-Transfer Processes in Transition-Metal-Doped Single-Crystal SrTiO3and TiO2

When SrTi${\mathrm{O}}_3$ or Ti${\mathrm{O}}_2$ doped with certain transition elements is irradiated with light in the region 0.39 to 0.43 μ, broad visible absorption bands with peak absorption as high as 100 ${\mathrm{cm}}^{-1\mathrm{}}$ are introduced. The study of optical and EPR spectra of Fe/Mo doped crystals under the influence of light and various heat treatments shows that the effect is due to a transfer of charge between Fe and Mo ions such that the nonabsorbing ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Mo}}^{6+}$ ions become ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Mo}}^{5+}$ with absorption bands in the visible.