Premelting and cation mobility in simple silicates: Contrasting the dynamics in the crystalline and molten state

Abstract The dynamics of atomic species in crystalline and molten sodium disilicate 2SiO 2 –Na 2 O is investigated and analyzed from molecular dynamics simulations. A clear signature of premelting is detected at T = T 0 ≈ 800 K that leads to a complete melting of the Na subnetwork while preserving the (Si,O) one. For higher temperatures, it is found that while the global behavior in terms of transport is rather similar between both states with values for the diffusivity and diffusion activation energies that are close, the nature of the ion motion is radically different. At low temperature, the motion of Na atoms in premelted crystals is, indeed, almost two-dimensional between layers of SiO 4/2 connected tetrahedra but with increasing temperature, the dynamics becomes essentially 3D. The analysis of the intermediate scattering function within the framework of trap models confirms the analysis.