EPR study ofNi+centers in SrF2

The EPR spectra of two kinds of ${\mathrm{Ni}}^{+}$ centers created by room-temperature x irradiation of Sr${\mathrm{F}}_2$: Ni single crystals are reported. One of them labeled as ${\mathrm{Ni}}^{+}$-I shows a pure tetragonal symmetry with a $g$ tensor given by the values $g_{\parallel }=2.597\mathrm{}\pm 0.005$ and $g_{\perp }=2.092\mathrm{}\pm 0.005$. The superhyperfine (SHF) structure is interpreted as due to the interaction with four equivalent fluorines. The interaction with each of these fluorines is given by an axial tensor with $A_{\parallel }=235\mathrm{}\pm 5$ MHz and $A_{\perp }=105\mathrm{}\pm 5$ MHz. The other type of ${\mathrm{Ni}}^{+}$ centers labeled as ${\mathrm{Ni}}^{+}$-II shows a slight orthorhombic distortion with respect to the tetragonal symmetry. The principal values of their $g$ tensor are $g_x=2.093\mathrm{}\pm 0.005$, $g_y=2.087\mathrm{}\pm 0.005$, and $g_z=2.589\mathrm{}\pm 0.005$ and the orientation of the principal axes is given by the Euler angles $\phi =45\mathrm{}°$, $\theta =2\mathrm{}°$, and $\Psi =0\mathrm{}°$ with respect to the cube axes. The observed SHF structure is interpreted in the same way as for ${\mathrm{Ni}}^{+}$-I centers but with $A_{\parallel }=250\mathrm{}\pm 5$ MHz and $A_{\perp }=100\mathrm{}\pm 5$ MHz. The basic model for both defects is a ${\mathrm{Ni}}^{+}$ ion displaced along one of the $〈100〉$ directions from the center of the cube of fluorines toward one of its faces, forming a kind of Ni${\mathrm{F}}_4^{3-}$ molecular ion. The slight orthorhombic distortion of ${\mathrm{Ni}}^{+}$-II centers is likely to be due to the presence of an extra defect in the neighborhood of ${\mathrm{Ni}}^{+}$ ions. The production and stability of both kinds of defects under different treatments is also reported. From these results and by comparison with optical absorption data we have concluded that ${\mathrm{Ni}}^{+}$-I centers are responsible for an absorption band at 270 nm and that ${\mathrm{Ni}}^{+}$-II centers produce an absorption band at 285 nm.