Crystal-clear liquid-liquid transition in a tetrahedral fluid

For a model known to exhibit liquid-liquid transitions, we examine how varying the bond orientational flexibility affects the stability of the liquid-liquid transition relative to that of the crystal phases. For very rigidly oriented bonds, the crystal is favored over all amorphous phase transitions. We find that increasing the bond flexibility decreases both the critical temperature T-c for liquid-liquid phase separation and the melting temperature T-m. The effect of increasing flexibility is much stronger for melting, so that the distance between T-c and T-m progressively reduces and inverts sign. Under these conditions, a naked liquid-liquid critical point bulges out in the liquid phase and becomes accessible, without the possibility of crystallization. These results confirm that a crystal-clear, liquid-liquid transition can occur as a genuine, thermodynamically stable phenomenon for tetrahedral coordinated particles with flexible bond orientation, but that such a transition is hidden by crystallization when bonds are highly directional.