Новая магнитная нецентральная примесь --- кобальт в титанате стронция

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 Å. 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).