“…The lines of bivalent iron, due to a very strong spin-phonon interaction, were resolved in EPR spectra only for the specimens cooled to liquid helium temperature. The isotropic lines with g = 4.3 and 2.02 are the characteristics of ESR spectra of various glasses with an impurity of isolated Fe 3+ ions, including aluminosilicate ones 50–53 . It is considered that these signals are associated with two types of Fe 3+ ions isolated in the glass framework: (1) g = 4.3 − ions in purely rhombic strong crystal field, E/D = 1/3, D >> hν (D, E − parameters of axial and rhombic component of crystalline fields, hν ∼ 10 GHz); (2) g = 2.0 - ions in a strong axial field E/D = 0).…”