Transport of Mobile Particles in an Immobile Environment: Computer Simulations of Sodium Silicates

Abstract

Molecular dynamics (MD) simulations of sodium silicate melts, (Na₂O)x(SiO₂) with x=2, 3, 20, are presented. In these systems, the mobility of sodium ions is much higher, often by orders of magnitude, than that of the silicon and oxygen atoms forming a tetrahedral network structure. We show that the high mobility of sodium is intimately related to the chemical ordering in sodium silicates. A network of percolating sodium-rich channels is formed in the static structure that serve as diffusion channels for the sodium ions. This channel network is revealed in static structure factors by a prepeak at the wavenumber q=0.95 A^-1. Inelastic neutron scattering experiments of sodium silicate melts, done at temperatures as high as 1600 K, confirm the existence of the latter prepeak. The channel diffusion of sodium ions yields a peculiar behavior of time-dependent density-density correlation functions, the so-called intermediate scattering functions. Whereas the incoherent scattering function for sodium, F^{s, Na}(q,t), detects the fast sodium ion diffusion, the coherent scattering function for Na-Na correlations, F_NaNa(q,t), decays on the slow time-scale of the Si-O matrix. This reflects the hopping motion of sodium ions between sodium sites, thereby F_NaNa(q,t) describing site-site correlations. Numerical calculations in the framework of mode coupling theory (MCT) are presented which use the partial static structure factors from the MD simulations as an input. The MCT results are in qualitative agreement with those from MD simulations.