Fragile-to-strong transitions in network glasses

Reference Presenter Authors
10-030 Pierre Lucas Lucas, P.(University of Arizona);

Glass forming liquids are commonly classified according to the temperature dependence of their viscosity following Angell’s fragility scale. Systems that exhibit an Arrhenius viscosity-temperature dependence such as SiO2 are classified as “strong” while those that show pronounced departure from Arrhenian behavior are dubbed “fragile”. Most glass forming liquids fall within these categories and the magnitude of the non-Arrhenius behavior is conveniently quantified using the fragility index “m” which reflects the steepness of the temperature-viscosity change near the glass transition temperature Tg. However, an increasing number of reports indicate that some glass forming liquids undergo a fragile-to-strong transition upon cooling, including substances as fundamental as water but also many metallic glasses. Here we show that network glasses including chalcogenide glasses also undergo such fragile-to-strong transitions. Interestingly many other fundamental physical properties such as density, heat capacity and conductivity also show abnormal behaviors during this transition. These effects may have direct implication to switching mechanism of phase change materials. The structural origin of these fragile-to-strong transition in tetrahedral network glass is also discussed in terms of topological principles.