Fragile-to-strong transition and polyamorphism in the energy landscape of liquid silica

Liquid silica is the archetypal glass former, and compounds based on silica are ubiquitous as natural and man-made amorphous materials. Liquid silica is also the extreme case of a 'strong' liquid, in that the variation of vis cosity with temperature closely follows the Arrhenius law as the liquid is cooled toward its glass transition temperature(1,2). In contrast, most liqu ids are to some degree 'fragile', showing significantly faster increases in their viscosity as the glass transition temperature is approached. Recent studies(3-6,35,36) have demonstrated the controlling influence of the poten tial energy hypersurface (or 'energy landscape') of the liquid on the trans port properties near the glass transition. But the origin of strong liquid behaviour in terms of the energy landscape has not yet been resolved. Here we study the static and dynamic properties of liquid silica over a wide ran ge of temperature and density using computer simulations. The results revea l a change in the energy landscape with decreasing temperature, which under lies a transition from a fragile liquid at high temperature to a strong liq uid at low temperature. We also show that a specific heat anomaly is associ ated with this fragile-to-strong transition, and suggest that this anomaly is related to the polyamorphic behaviour of amorphous solid silica.