  The local environment and the charge state of a nickel impurity in cubic
Ba$_{0.8}$Sr$_{0.2}$TiO$_3$ are studied by XAFS spectroscopy. According to the
XANES data, the mean Ni charge state is $\sim$2.5+. An analysis of the EXAFS
spectra and their comparison with the results of first-principle calculations
of the defect geometry suggest that Ni$^{2+}$ ions are in a high-spin state at
the $B$ sites of the perovskite structure and the difference of the Ni$^{2+}$
and Ti$^{4+}$ charges is mainly compensated by distant oxygen vacancies. In
addition, a considerable amount of nickel in the sample is in a second phase
BaNiO$_{3-\delta}$. The measurements of the lattice parameter show a decrease
in the unit cell volume upon doping, which can indicate the existence of a
small amount of Ni$^{4+}$ ions at the $B$ sites.
