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

• We reproduce a full melting of a sodium disilicate crystal and its quench to the glass.
• The dynamics of both states shows marked differences.
• In the premelted state, the Na motion evolves from 2D to 3D.
• The ordering of the (Si,O) network affects the glassy relaxation in the premelted state.

The dynamics of atomic species in crystalline and molten sodium disilicate 2SiO2–Na2O is investigated and analyzed from molecular dynamics simulations. A clear signature of premelting is detected at T = T0 ≈ 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 SiO4/2 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.