Excess entropy scaling of transport properties in network-forming ionic melts (SiO $_2$2 and BeF $_2$2)

Abstract

The regime of validity of Rosenfeld excess entropy scaling of diffusivity and viscosity is examined for two tetrahedral, network-forming ionic melts (BeF(2) and SiO(2)) using molecular dynamics simulations. With decrease in temperature, onset of local caging behavior in the diffusional dynamics is shown to be accompanied by a significant increase in the effect of three-body and higher-order particle correlations on the excess entropy, diffusivity, ionic conductivity, and entropy-transport relationships. The signature of caging effects on the Rosenfeld entropy scaling of transport properties is a distinctly steeper dependence of the logarithm of the diffusivity on the excess entropy in ionic melts. This is shown to be true also for a binary Lennard-Jones glassformer, based on available results in the literature. Our results suggest that the onset of a landscape-influenced regime in the dynamics is correlated with this characteristic departure from Rosenfeld scaling. The breakdown of the Nernst-Einstein relation in the ionic melts can also be correlated with the emerging cooperative dynamics.