Cobalt in strontium titanate as a new off-center magnetic impurity

The local structure and charge state of the cobalt impurity in SrTiO$_3$ is studied by X-ray absorption fine structure (XAFS) spectroscopy. The synthesis conditions, under which cobalt predominantly (up to 76%) substitutes the atoms at the $A$ site of the perovskite structure, is found for SrTiO$_3$(Co) samples. By varying the synthesis conditions, it is possible to appreciably change the ratio between the concentrations of cobalt atoms incorporated into the $A$ and $B$ sites. It is established that the charge state of cobalt is +2 at the $A$ site and +3 at the $B$ site. It is revealed that the Co impurity at the $A$ site is off-center, and its deviation from the lattice site is 1.0 $\mathring{\mathrm{A}}$. First-principles calculations show that the isolated Co$^{3+}$ ion at the $B$ site is diamagnetic, whereas the Co$^{2+}$ ion at the $A$ site is in a high-spin state ($S$ = 3/2).