Abstract

Molecular Dynamics (MD) simulations were used to study the relationship between structure and transport properties in five liquids (NaAlO 2 , Na 2 Al 2 SiO 6 , NaAlSiO 4 , NaAlSi 2 O 6 , and NaAlSi 3 O 8 ) in the system NaAlO 2 -NaAlSi 3 O 8 at temperatures ranging from 4000 to 6000 K and pressures from 0 to 55 GPa. Seventy simulations were carried out in the microcanonical ensemble using a simple pair-wise additive potential with Coulombic interaction and Born-Mayer repulsion. Detailed study of the coordination of O and network forming cations provides a master set of coordination environment or speciation curves. These master curves were applicable to all compositions and temperatures and were most explicit when compression (V r /V; where V r is the molar volume at a reference pressure) was used as the independent variable. The universality implied that coordination environments for network atoms O, Al, and Si depend weakly upon Si/Al, T/O, or Na/T atomic ratios for the compositions studied. Self-diffusion coefficients, computed from analysis of mean-square displacements, were used to evaluate the activation enthalpy (H a = E a +PV a ) for self-diffusion for each species. The activation energy (E a ) for Na was independent of composition, whereas E a for O, Si, and Al increased as Si/Al increased. Activation volume (V a ) at pressure <15 GPa was positive for Na and negative for O, Si, and Al and decreased with increasing Si/Al for all species. An extension of the Adam-Gibbs-DiMarzio configurational entropy theory taking explicit account of (super [2]) O and (super [3]) O mixing explained both the variation of the pressure-derivative of the shear viscosity as a function of composition and the disappearance of "anomalous" viscosity behavior at P> approximately 25 GPa for all compositions in the system NaAlO 2 -NaAlSi 3 O 8 .

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