Transport properties of tetrahedral, network-forming ionic melts

Molecular dynamics simulations of liquid silica and beryllium fluoride are performed using the van Beest-Kramer-van Santen and transferable rigid ion model potentials, respectively, in order to compare transport properties. The ionic conductivity (sigma), shear viscosity (eta) and ionic self-diffusivities (D(+/-)) are computed over a fairly wide range of temperatures and densities and deviations from Arrhenius behavior along different isochores is studied. The Stokes-Einstein relation is shown to hold over the entire range of state points, though the effective hydrodynamic radius shows small variations due to thermal fluctuations, compression, and local tetrahedral order. Several alternative tests of the Nernst-Einstein relation are implemented which show that significant network-formation in the anomalous regime leads to a breakdown of this relationship. The relaxation times, tau(sigma) and tau(M), associated with the decay of the charge-flux and pressure ACFs respectively, are computed. In the anomalous regime, as the tetrahedral network formation progresses, tau(M) increases rapidly while tau(sigma) shows very little variation, indicating a decoupling of charge and momentum transport processes.