Molecular dynamics simulations of liquids and glasses in the system NaAlSiO4-SiO2; methodology and melt structures

Abstract

This is the first of a two-part molecular dynamics (MD) study that examines the effects of temperature, pressure, and composition on the structure and properties of ten compositions in the system NaAISiO.-Si02. Results were obtained for collections of at least 1300 atoms at temperatures between 2500 and 4500 K, pressures of 2-5 GPa, and simulation durations on the order of 0.1 ns. Durations and numbers of particles are both about three times larger than for previous MD simulations on molten aluminosilicates. This study addresses experimental matters, including aspects of simulation methodology that affect the accuracy of atom trajectories (hence computed properties). These include Nt (total number of atoms in the MD box) and spatial resolution achieved in the interparticle force calculation. Static melt structures and their systematic variation with temperature and composition are also explored. In the second part (Stein and Spera, in preparation), the mechanism of diffusion is studied in detail, and MD-computed results for a variety of thermodynamic and transport properties are reported and related explicitly to melt structure. The present study employs a pairwise-additive form of the interatomic potential energy function, using the parameters of Dempsey and Kawamura (1984) with electrostatic interactions computed by the Ewald sum technique. Greater than 90% of Si and AI are fourfold-coordinated by 0 and >90% of 0 is in twofold coordination by the tetrahedral (T) cations, showing little change with composition and temperature. T -T coordination is found to have a more distinct dependence on composition than does T -0 coordination. Structures and properties determined with this set of parameters differ from those computed using the parameters published by Angell et al. (1987), which give a less strongly ordered tetrahedral network and rates of diffusion for network-forming ions that are larger by about an order of magnitude. Results indicate that transport properties computed from time-correlation functions (e.g., diffusivity and ionic conductivity) apparently become asymptotic for system sizes (Nt) greater than about 1000 particles. Relative to these values, properties computed with Nt less than about 400-600 particles are generally overestimated by 10-100% and show increased variance. Truncation of the Ewald sum (limiting the number of reciprocal space vectors) introduces additional variance in computed properties. Melt structure (e.g., nearest-neighbor coordination statistics, static pair-correlation functions, and intertetrahedral bridging angle distributions) are less dependent upon system size than transport properties. The particular form and parameterization ofinterionic potential influence computed properties much more than does system size or spatial resolution in the force calculation. Substitution of (Na + AI) for Si is accompanied by a decrease in the mean intertetrahedral bridging (T-0- T) angle related to increasing numbers of AI-O-AI bridges and produces broader and less sharply peaked angle distributions. Radial density functions computed from the MD configurations reveal muted structure beyond the T -T 1 coordination shell in comparison with published XRD analyses, although there is evidence of structure due to T-02 and T-T2 correlation near 5 A, especially in the simulation of molten NaAlSiO.. Structures produced by the MD model reflect the effects of using simplistic potential energy functions, as well as characteristically high experiment temperatures and rapid quench rates required by the practical limitations of the MD method.