Oxygen vacancies in BaTiO3

Using electron paramagnetic resonance two types of ${\mathrm{Ti}}^{3+}$-related centers are identified in BaTi${\mathrm{O}}_3$, which are attributed to ${\mathrm{Ti}}^{3+}$-$V_{\mathrm{O}}$ and to ${\mathrm{Ti}}^{3+}$-$V_{\mathrm{O}}$ 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 structure, resolved for ${\mathrm{Ti}}^{3+}$-$V_{\mathrm{O}}$-Na(K). The symmetry of the centers is broken in the sense that an electron is localized near one of the two equivalent ${\mathrm{Ti}}^{4+}$ ions next to $V_{\mathrm{O}}$. Also the observed Ti hyperfine structure points to electron capture at one Ti site. The $V_{\mathrm{O}}$-type centers are contrasted to isolated ${\mathrm{Ti}}^{3+}$, representing a free small polaron in crystals containing ${\mathrm{Nb}}^{5+}$ and no oxygen vacancies. The participation of the $V_{\mathrm{O}}$ centers in photochromic processes in BaTi${\mathrm{O}}_3$ 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 an exciton bound to the $V_{\mathrm{O}}$-Na(K) complex.