Paramagnetic centers of 3 types were found in SrF2:Fe(0.2at%) crystals. Two types were observed in the untreated crystals, and the third type appears only in the crystals irradiated by X-rays. The electron paramagnetic resonance spectra of one type of centers in a non-irradiated crystal and of the centers that appear after irradiation were described by the orthorhombic Hamiltonians with an effective spin Seff = 5/2. In both cases, the centers were observed at 4.2 and 77K. The principal axes of the spin Hamiltonians for them were along the 〈001〉, 〈11¯0〉 and 〈110〉 axes. However, the fine-structure parameters of their electron paramagnetic resonance spectra differ significantly. An analysis of the superhyperfine structure of the electron paramagnetic resonance spectra shows that the radiation center forms through substitution of a Fe2+ ion for a Sr2+ cation. Under X-ray irradiation, the Fe2+ ion transforms into the Fe3+(6A1g ) state and was displaced to an off-center position along the C2 axis of its coordination cube. The absence of a superhyperfine structure in the electron paramagnetic resonance spectra of the orthorhombic centers in a non-irradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model was proposed for this structure. The electron paramagnetic resonance spectra of centers of the third type were observed only at 4.2K, and the structure of these centers was not studied.

EPR of Trivalent Iron Centers in SrF2:Fe Crystals. E.R.Zhiteĭtsev, V.A.Ulanov, M.M.Zaripov: Physics of the Solid State, 2007, 49[5], 845-50