Figure 5
Molecular dynamics simulation results for mixed alkali compensated alumino-silicate glasses and melts. (A) Total and partial pair distribution functions ρ(r) of glasses at 300 K, highlighting Si-O, Al-O, Na-O and K-O in [Na0.5K0.5]AlSiO4 glass. (B) ρ(r) for network cations for Al/Si = 1 glasses with different X K , showing inter-tetrahedral distances increasing with X K between 3.0 and 3.5 Å, 18 % Al-Al correlations averaged over all TO4 units. (C) Vibrational Density of States (Supplementary Information Methods) calculated for Al/Si = 1 glasses with X K = 0.0, 0.25, 0.75, 1, increasing vertically and replicating features and trends found in Raman spectra (Fig. 3). (D) Experimental Molar Volumes per TO4 tetrahedron as a function of X K for glasses with Al/Si = 0.33, 0.72 and 1, in excellent agreement with MD predictions. (E) Alkali total volume as a function of the non-network volume ξ (Supplementary Information), including the percolation threshold at ξ C = 0.25. ξ C is reached at X K ~ 0.7 for Al/Si = 1, and at X K ~ 1 for Al/Si = 0.66. (F) Elemental diffusivities D i from MD calculations for all 15 melts at 2000 K obtained from 50 ns MSD calculations (Supplementary Information). (G) Cluster sizes, determined from alkali volume fractions shown in Fig. 5E, represented as a function of X K together with the viscosities η at 2000 K (calculated from the D i values, see Supplementary Information Fig. S7) for 15 different melts (Al/Si = 0.33, 0.6 and 1; X K = 0, 0.25, 0.5, 0.75). Viscosities are normalized to the viscosity for X K = 0. Variations of viscosity and cluster size are clearly correlated, both presenting super-exponential behavior.
