Propagation of the polyamorphic transition of ice and the liquid-liquid critical point

Water has a rich metastable phase behaviour that includes transitions between high- and low-density amorphous ices, and between high- and low-density supercooled liquids. Because the transitions occur under conditions where crystalline ice is the stable phase, they are challenging to probe directly. In the case of the liquids, it remains unclear(1) whether their mutual transformation at low temperatures is continuous(2,3), or discontinuous(4,5) and terminating at a postulated second critical point of water that is metastable with respect to crystallization. The amorphous ices are more amenable to experiments(6-8), which have shown that their mutual transformation is sharp and reversible. But the non-equilibrium conditions of these studies make a firm thermodynamic interpretation of the results difficult. Here we use Raman spectroscopy and visual inspection to show that the transformation of high- density to low-density amorphous ices involves the propagation of a phase boundary-a region containing a mixture of both ices. We find that the boundary region becomes narrower as the transformation progresses, and at higher transformation temperatures. These findings strongly suggest that the polyamorphic ice transition is discontinuous; a continuous transformation should occur uniformly over the entire sample(9). Because the amorphous ices are structurally similar to their supercooled liquid counterparts, our results also imply that the liquids transform discontinuously at low temperatures and thus support the liquid-liquid critical-point theory(4,5).