Paramagnetic defects in neutron-irradiated phenakite crystals

Radiation-disordered crystals of phenakite Be$_2$SiO$_4$ (at fast neutron fluences of (0.75, 1.0, 1.2, 6.6, 8.5) $\times$ 10$^{18}$ cm$^{-2}$) have been investigated using EPR spectroscopy. It has been established that the main radiation damages predominantly occur in the silicon-oxygen sublattice of the crystal. Vacancy centers ($E'$ centers, $g$ = 2.0015), defects of the displaced oxygen type, and O$^-$ hole-type centers have been revealed. The paramagnetic absorption signal with the parameters $g_x$ = 2.0218, $g_y$ = 2.0124, and $g_z$ = 2.0027 has been identified with the [SiO$_4$]$^{3-}$ complex (a variety of O$^-$ centers). The intense EPR signals with the parameters $g_x$ = 2.0526, $g_y$ = 2.0020, $g_z$ = 2.0066 and $g_x$ = 2.0290, $g_y$ = 2.0030, $g_z$ = 2.0099 have been assigned to two types of O$_2^-$ molecular ions with different local environments. The theoretical models of radiation-induced centers have been discussed by comparing the EPR and optical spectroscopic data.