Fragile-to-strong crossover and polyamorphism in liquid silica: changes in liquid structure

Abstract

Previous numerical investigations of model silica have shown evidence favouring liquid–liquid phase separation in the deeply supercooled liquid. This thermodynamic instability is argued to be responsible for the polyamorphic behaviour in the glass. In these previous investigations, a method for discerning local density was developed by writing the radial distribution function as a sum of contributions from neighbours of a given atom, sorted in order of their distance from the given atom. Quite separately, along isochores of densities comparable with that of silica at ambient pressure, simulations have also shown that the Arrhenius dependence of diffusivity on temperature T so characteristic of liquid silica breaks down at higher temperatures. Indeed, evidence indicates that silica approaches Arrhenius (strong) behaviour upon cooling from higher temperatures where it exhibits super-Arrhenius (fragile) behaviour. We now revisit this fragile-to-strong crossover using the technique of discerning density differences developed in the polyamorphism study. We show that the technique is useful in illustrating the structural change taking place through the crossover. We also see that the structures present in the high-temperature liquid reappear upon increasing density, supporting the observation that fragility increases upon compression of the liquid. Furthermore, we see that these same structures are typical of those which presage the liquid–liquid phase separation at lower temperatures.

Acknowledgements

P.H.P. and I.S.-V. wish to thank Natural Sciences and Engineering Research Council (Canada) for funding. F.S. wishes to thank Ministerio dell 'Istruzione, dell Università e della Ricerca (MIUR), Progretti di Ricerca di rilevante Interesse Nationale (PRIN) COFIN and Fondo per gli Investimenti della Ricerca di Base (FIRB) for support.