In Situ High-Temperature Probing of the Local Order of a Silicate Glass and Melt during Structural Relaxation

Direct,observation of structural, relaxation at molecular scale in network glasses near the glass transition and in the melt is very challenging. Distribution of structural units forming the glass is commonly believed to depend uniquely on composition and temperature. In this paper, we evidence the dynamical structural changes of the silicate network upon structural relaxation by monitoring the temperature and tithe evolution of its vibrational, signature. just above T-g, the silicate network presents an unexpected time dependence of its relaxation process;characterized by two subsequent regimes leading to a disordered equilibrium state. When annealed at 925 K, the network evolves rapidly toward silica and sodium silicate crystalline phases. Evolution models to describe the temperature time dependences of the Q(n) distributions are reported and discussed. A general picture of the hierarchical character of structural relaxation in network glasses is drawn.