Identification of Paramagnetic Centers in Irradiated F-Doped Silica by First-Principle Calculations

Although fluorine is one of the most important dopants of silica, for several decades fluorine-related paramagnetic defects have been very elusive. Yet, a recent experimental investigation [L. Skuja et al. Phys. Rev. Lett. 131, 256 903 (2023)] has claimed the identification of a new paramagnetic center, that is, the E′(F) center, responsible for a 10.5 mT hyperfine doublet in irradiated F-doped silica glass. Due to the lack of theoretical investigations, the reliability of the assignment proposed for the E′(F) has been under debate so far. In this work two alternative structural models, here labeled as the 2fSi-F and E'_F models, are investigated and discussed by means of first-principle calculations of electron paramagnetic resonance parameters, and further evidence in favor of the 2fSi-F model is provided. The latter consists of a threefold-coordinated Si atom, which is bonded to two bridging oxygen atoms and to one fluorine atom and that features an unpaired electron in an sp³ orbital. Furthermore, through first-principles nudged elastic band calculations it is shown that, under irradiation, generation mechanisms involving an interconversion between E'_gamma and E′(F) centers can occur, with no need for precursors like the tetrahedral unit SiO₂F₂ or twofold Si centers.