Fragile-to-strong transitions in glass forming liquids

Since they were first reported almost two decades ago, fragile-to-strong (FTS) transitions have been observed in all categories of glass-forming systems including metallic, covalent, ionic and molecular liquids. In these systems, a transition from Arrhenius to non-Arrhenius viscosity behavior is observed in conjunction with anomalies in multiple thermodynamic variables including the heat capacity, the thermal expansion coefficient and the isothermal compressibility. A review of experimental evidences supporting the existence of FTS transitions is presented. In particular FTS transitions are found to be concomitant with a peak in heat-capacity as well as density extrema associated with a negative expansion coefficient in the region of the transition. Moreover, spectroscopic analyses also show distinct structural changes across these liquid-liquid transitions. Reports of FTS transitions in each category of liquid are reviewed.