Controlled oxygen vacancies and space correlation with Cr³⁺ in SrTiO₃

By means of electron paramagnetic resonance (EPR) measurements on thermally reduced Cr-doped SrTiO₃, we find a strong correlation between the Cr³⁺ ions and oxygen vacancies (V_os). A charge compensation associated with the creation of V_os induced by thermal reduction causes the valence of the Cr ions to change from 4+ to 3+. This induces a symmetry change in the crystal field experienced by the Cr atom from cubic to axial. On the other hand, in Cr/Nb-codoped samples, where V_os are absent, the Cr³⁺ EPR signal is isotropic, indicating that these Cr ions occupy an octahedral site of cubic symmetry. From the angular dependence of the EPR spectra we determine an axial crystal field parameter (D≃8 MHz), which is consistent with a perturbation induced by a V_o localized at the third coordination sphere of Cr³⁺. This is in contrast to other transition-metal dopants, where the V_o is located at the first coordination sphere. Thus we identify a linear complex of the form Cr³⁺-O-Ti³⁺-V_o-Ti³⁺, which is unique in its structure, and which, furthermore, might account for the stabilization of the insulating state in reduced Cr-doped SrTiO₃ crystals.