Annealed high-density amorphous ice under pressure

The well-known expansion of water on cooling below 277 K is one of several peculiar properties that could signal a second critical point near 220 K and 0.1 GPa in pressure, deep in the supercooled liquid phase. Evidence for this would be a first-order transition line between two distinct supercooled liquids at temperatures below the critical point. As that lies below the minimum crystallization temperature, experimental tests have instead used low- and high-density amorphous ices-LDA and HDA-as proxies for the supercooled liquids. But numerous studies over the past decade have not yielded a clear consensus about the nature of the HDA/LDA transition. Here we identify a previously uncharacterized state of high-density amorphous ice obtained if HDA is annealed at pressures near 2 kbar. The transition between this annealed HDA and LDA is strikingly different from the behaviour found in earlier work, in a way that favours the two-liquid model.