Formation and annihilation of point defects in SiO2 glass during neutron irradiation and annealing

Abstract

We perform systematic molecular dynamics (MD) simulations to explore the evolution of point defects in SiO₂ glass during neutron irradiation and annealing. Our results suggest that the numbers of point-defects nonbridging oxygens (NBO) and E′-type center increase significantly within 0.2 ps, and then decrease gradually over 0.2 ps. The numbers of point-defects oxygen deficient centers (ODC), peroxy linkages (POL) and peroxy radicals (POR) present gradual increase tendency within 0.2 ps. The number of these point defects is in direct proportion to the kinetic energies of the primary knock-on atoms. During annealing, we consider the situations that SiO₂ glass is irradiated by neutron at 1000 K, 1500 K and 2000 K. It is found that point defects disappear quickly during annealing and it qualitatively agrees with the literature results, where E’, ODC and NBOs in irradiated silica glasses are bleached when annealing at temperatures exceeding 550 °C. Finally, we consider the vibrational density of states of SiO₂, and find that the low-frequency (0.1∼0.5 THz) vibrational density of states is greatly enhanced.