Photochromism in Transition-Metal-Doped SrTiO3

The photochromic properties of transition-metal-doped SrTi${\mathrm{O}}_3$ are studied using optical and electron paramagnetic resonance techniques. In addition to the double doped systems containing Fe + Mo, and Ni + Mo, which were treated briefly in earlier publications, the photochromic effect in SrTi${\mathrm{O}}_3$ singly doped with Ni, Fe, and Co is discussed in detail. The centers responsible for the photochromic absorption are identified, and the visible absorption bands are tied to specific transition metals in particular sites (axial or cubic) and oxidation states. It is found that when Mo is not present, a transition ion can act as both an electron donor and an electron trap. Models are proposed for the nature of the band-edge coloring absorption and the visible photochromic absorption bands.