Structure and dynamics in network liquids
The onset of the Glass Transition in tetrahedral network liquids is associated with the over-constrained nature of the structure and the low ability of the ions to move relative to one another. We investigate the interplay between the structure and dynamics in BeF2, a template for the ideal tetrahedral system. We see that the ionic diffusion coefficients can be predicted from the calculated viscosity of the system using the Eyring hopping model of diffusion, with a diffusive jump length approximately corresponding to the radius of the first coordination sphere. Novel correlation functions are developed which enable us to identify the events responsible, on an atomistic level, for the structural rearrangements which correspond to the barrier crossing in this hopping model of diffusion, and we find that these events can be identified as the exchange of ions in the local coordination poly- hedra, or cage, of the cations. The calculation of the rate of the decay of these cages allows us to predict the macroscopic diffusion coefficients with the definition of a jump length over which the diffusive hops occur, and to scale the behaviour of the system at different temperatures by setting the cage lifetime as an effective clock for the system. Comparison between simulations performed with and without the inclusion of the effects of anion polarisation suggest that the polarisation plays an important role in the ability of the system to undergo the cage decay events and to create the defect sites which facilitate a decrease in the number of constraints acting in the system. The decay of the cages describes the local rearrangement of the ions in the first coordi- nation shell of a given ion. The development of other correlation functions allows us to investigate the spatial relationship between these cage decay events over longer length and time scales, and also to investigate how the local structure of the first coordination shells of the cations relates to their ability to undergo the cage decay events and to form the defects. These functions are then used to investigate the link between the structure and the dy- namics in some molten trichloride systems, which have different network structures, and hence a different relationship between the cage decay and the diffusion. Finally, we investigate the effect of changing the potential parameters in BeF2, and we find that the effective polarisability of the system can be controlled such that a less diffusive system may be described, giving a good representation of both structural and dynamical experimental data.