EPR study ofNi+andNi3+in x-irradiated CaF2

Abstract

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 ${g}_{\ensuremath{\parallel}}=2.569$ and ${g}_{\ensuremath{\perp}}=2.089$ 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 ${(\mathrm{N}\mathrm{i}{\mathrm{F}}_{4})}^{3\ensuremath{-}}$ molecular ion with the ${\mathrm{Ni}}^{+}$ at a distance of about 0.37 \AA{} 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 ($g=2.003$) 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.