A STRONG TO FRAGILE TRANSITION IN A MODEL OF LIQUID SILICA

The transport properties of an ionic model for liquid silica [1] at hi gh temperatures and pressure are investigated using molecular dynamics simulations. With increasing pressure, a clear change from ''strong'' to ''fragile'' behaviour (according to Angell's classification of gla ss-forming liquids) is observed, albeit only an the small viscosity ra nge that can be explored in MD simulations. This change is related to structural changes, from an almost perfect four-fold coordination to a n imperfect five or six-fold coordination.