Site-selective spectroscopy of defect chemistry in ${\mathrm{SrTiO}}_3$, ${\mathrm{Sr}}_2$${\mathrm{TiO}}_4$, and ${\mathrm{Sr}}_3$${\mathrm{Ti}}_2$${\mathrm{O}}_7$

${\mathrm{Eu}}^{3+}$-doped ${\mathrm{SrTiO}}_3$, ${\mathrm{Sr}}_2$ ${\mathrm{TiO}}_4$, ${\mathrm{Sr}}_3$ ${\mathrm{Ti}}_2$ ${\mathrm{O}}_7$, and ${\mathrm{Sr}}_4$ ${\mathrm{Ti}}_3$ ${\mathrm{O}}_{10}$ phases have been studied by site-selective laser spectroscopy. The spectra of the dominant sites have been identified and assigned to ${\mathrm{Eu}}^{3+}$ substitution on Sr sites with distant charge compensation. The spectra of nonstoichiometric samples show that the nonstoichiometry is accommodated by formation of Ruddlesden-Popper phases. There is no evidence that appreciable numbers of point defects accompany the nonstoichiometry. Additional centers are observed when the dopant concentration is raised. These centers are assigned to locally compensated ${\mathrm{Eu}}^{3+}$ sites and/or clusters of several Eu dopants where point defects provide local compensation.