Abstract
Electron-paramagnetic-resonance conversion-rate measurements combined with optical-absorption data in Ni-doped SrTiO3 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 Ni3+ to become Ni4+. The free electrons created are trapped at Ni3+-Vo centers which convert to Ni3+-Vo+e. Acceptor charge transfer at near-band-gap energy λ<500 nm generates Ni3+-Vo+2e centers from the latter. The holes liberated are trapped at cubic Ni2+ sites which convert back to Ni3+. The D2 band at 575 nm is a Ni3+-Vo+2e donor band which changes this center back to the Ni3+-Vo+e state. The optical transfer rates measured for the creation and quenching of the Ni3+-Vo+2e center verify the above-described mechanism. © 1976 The American Physical Society.