Initial stage of physical ageing in network glasses

An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (T_g ) in network glasses is developed for the archetypal chalcogenide glass, As₂₀Se₈₀, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below T_g . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.