On the nature of change in Ni oxidation state in BaTiO3-SrTiO3 system
  XAFS studies of Ni-doped Ba$_{1-x}$Sr$_x$TiO$_3$ solid solution reveal that the Ni oxidation state changes from 4 in SrTiO$_3$ to 2.5 in BaTiO$_3$ when varying $x$. This change is accompanied by a noticeable change in the interatomic Ni-O distances in the first shell. The first-principles calculations show that nickel creates an impurity band in the forbidden band gap of BaTiO$_3$ and SrTiO$_3$, which explains the appearance of intense absorption of Ni-doped samples in the visible region. The analysis of the electronic structure of doped crystals and calculations of the oxygen vacancy formation energy in them show that different oxidation states of Ni in SrTiO$_3$ and BaTiO$_3$ can be explained by different formation energies of the oxygen vacancies in these compounds. 
A. I. Lebedev and I. A. Sluchinskaya
cond-mat.mtrl-sci