New stabilized VK-type center in electron-irradiated sodium fluoride observed by electron paramagnetic resonance

Sodium fluoride crystals were bombarded with MeV electrons near room-temperature or at 21K and their EPR spectra measured. While after RT-irradiation one observes the spectrum of the classical cubic F-center, the low-T irradiation introduces a new low-symmetry defect which transforms reversibly into the F-center above ≈200K. We suggest for the new center a triplet VK-type defect stabilized during irradiation by the nearby created F-center. While the ordinary VK-center as observed after low-T X-ray and γ-irradiation vanishes at 200K the present electron-induced defect is stable and reappears upon cooling; it is still discernible after anneals up to 200°C.