By means of electron paramagnetic resonance (EPR) measurements on thermally reduced Cr-doped ${\text{SrTiO}}_3$, we find a strong correlation between the ${\text{Cr}}^{3+}$ ions and oxygen vacancies $\left({\text{V}}_{\text{o}}\text{s}\right)$. A charge compensation associated with the creation of ${\text{V}}_{\text{o}}\text{s}$ 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 ${\text{V}}_{\text{o}}\text{s}$ are absent, the ${\text{Cr}}^{3+}$ 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 $\left(D\simeq 8\text{MHz}\right)$, which is consistent with a perturbation induced by a ${\text{V}}_{\text{o}}$ localized at the third coordination sphere of ${\text{Cr}}^{3+}$. This is in contrast to other transition-metal dopants, where the ${\text{V}}_{\text{o}}$ is located at the first coordination sphere. Thus we identify a linear complex of the form ${\text{Cr}}^{3+}{\text{-O-Ti}}^{3+}{\text{-V}}_{\text{o}}{\text{-Ti}}^{3+}$, which is unique in its structure, and which, furthermore, might account for the stabilization of the insulating state in reduced Cr-doped ${\text{SrTiO}}_3$ crystals.

• Received 4 June 2009

DOI:https://doi.org/10.1103/PhysRevB.80.075122