Photochromism in Ni-doped SrTiO3

Electron-paramagnetic-resonance conversion-rate measurements combined with optical-absorption data in Ni-doped ${\mathrm{SrTiO}}_3$ have been carried out. From these experiments in oxidized and reduced crystals, assignment of the photochromic bands in the visible could be made and the trapping mechanism identified. The ($B$) band at 525 nm is due to donor transfer of cubic ${\mathrm{Ni}}^{3+}$ to become ${\mathrm{Ni}}^{4+}$. The free electrons created are trapped at ${\mathrm{Ni}}^{3+}-V_o$ centers which convert to ${\mathrm{Ni}}^{3+}-V_o+e$. Acceptor charge transfer at near-band-gap energy $\lambda <500\mathrm{}$ nm generates ${\mathrm{Ni}}^{3+}-V_o+2e$ centers from the latter. The holes liberated are trapped at cubic ${\mathrm{Ni}}^{2+}$ sites which convert back to ${\mathrm{Ni}}^{3+}$. The $D_2$ band at 575 nm is a ${\mathrm{Ni}}^{3+}-V_o+2e$ donor band which changes this center back to the ${\mathrm{Ni}}^{3+}-V_o+e$ state. The optical transfer rates measured for the creation and quenching of the ${\mathrm{Ni}}^{3+}-V_o+2e$ center verify the above-described mechanism.