A new scenario of the apparent fragile-to-strong transition in tetrahedral liquids: water as an example

dRecently it was suggested that tetrahedral liquids such as water and silicon exhibit a fragile-to-strong transition while approaching the glass-transition temperature (T-g). Such a drastic change in the fragility of liquid as a function of temperature is very rarely observed. Here we propose that, contrary to the popular fragility transition scenario, this phenomenon should be explained in terms of the crossover from a non-glass-forming to a glass-forming branch. For ordinary glass-forming liquids, there is frustration between long-range density ordering (crystallization) and short-range bond ordering (energetic frustration hidden in the interaction potential), which helps vitrification. For water and silicon, such frustration does not exist near the melting point (T-m) at ambient pressure since the symmetry of their crystals:is consistent with that of short-range tetrahedral bond ordering. Thus, we call this high temperature region near T-m a non-glass-forming branch. In the low-temperature region near T-g, which we call a glass-forming branch, on the other hand, a system tends to have long-range density ordering. Its competition with local tetrahedral ordering induces strong frustration effects, which make the liquid strong. Our scenario suggests that the crossover from,a non-glass-forming to a glass-forming branch may be generic to tetrahedral liquids whose specific volume increases upon crystallization.