OXYGEN VACANCIES IN BATIO3

Using electron paramagnetic resonance two types of Ti3+-related center s are identified in BaTiO3, which are attributed to Ti3+-V-O and to Ti 3+-V-O associated with Na or K, respectively. This assignment is based on the chemical and illumination conditions leading to the detection of the defects. It is furthermore supported by the Ti hyperfine struct ure, resolved for Ti3+-V-O-Na(K). The symmetry of the centers is broke n in the sense that an electron is localized near one of the two equiv alent Ti4+ ions next to V-O. Also the observed Ti hyperfine structure points to electron capture at one Ti site. The V-O-type centers are co ntrasted to isolated Ti3+, representing a free small polaron in crysta ls containing Nb5+ and no oxygen vacancies. The participation of the V -O centers in photochromic processes in BaTiO3 is studied. In crystals doped with Na or K an optical absorption precursor of the fundamental absorption is identified and attributed to the creation of ail excito n bound to the V-O-Na(K) complex.