EPR study ofNi+andNi3+in x-irradiated CaF2

EPR measurements of Ca${\mathrm{F}}_2$:Ni crystals before and after room-temperature (RT) x irradiation are reported. Depending on the rare-earth impurity content a small concentration of ${\mathrm{Ni}}^{+}$ centers is observed in "as-grown" crystals. During RT x irradiation the ${\mathrm{Ni}}^{+}$ concentration increases. At least two different kinds of ${\mathrm{Ni}}^{+}$ centers are observed in the EPR spectra. One of them has tetragonal symmetry with $g_{\parallel }=2.569\mathrm{}$ and $g_{\perp }=2.089\mathrm{}$ and gives a superhyperfine (SHF) structure, due to the interaction with four equivalent flouride ions, which can be resolved at liquid-nitrogen temperature (LNT). The other one has an almost tetragonal symmetry with $g$ factors and SHF structure very close to the former one but the (SHF) structure is resolved at RT. The first ${\mathrm{Ni}}^{+}$ center is associated with a ${\left(\mathrm{N}\mathrm{i}{\mathrm{F}}_4\right)}^{3-}$ molecular ion with the ${\mathrm{Ni}}^{+}$ at a distance of about 0.37 Å from the plane of the fluorine atoms. The other EPR signal is assigned to a similar center with some kind of perturbation. An isotropic EPR signal ($g=2.003\mathrm{}$) with a well-resolved superhyperfine structure due to the interaction with eight equivalent fluorines has been detected in some of the samples after RT x irradiation. The tentative model proposed for the center responsible for this signal is a ${\mathrm{Ni}}^{3+}$ ion at a ${\mathrm{Ca}}^{2+}$ substitutional position.