The Transformations and Nature of Amorphous Ices

The amorphous forms of water are of considerable importance and interest. Ice was the system in which pressure amorphisation was first discovered and is a model for the understanding of this phenomenon. The amorphous ices provide our principal experimental test of the two liquids model of water which seeks to explain the unusual properties of liquid water by the existence of critical point and a liquid-liquid phase transition in the inaccessible supercooled region of the phase diagram. Finally, the transformations between the various amorphous ices provide a means to explore the phenomenon of amorphous-amorphous transitions.

There are three amorphous ices, low-density amorphous ice (LDA) which is formed by rapid cooling of the liquid at ambient pressure, high-density amorphous ice (HDA) which is formed by compressing ice Ih at 77 K and very high density amorphous ice (VHDA) which is formed by warming HDA to ~130 K at pressures above 0.8 GPa. Furthermore 'as-formed' HDA anneals on warming below 0.8 GPa and this annealed subvarient (e-HDA) behaves significantly differently from the unrelaxed 'as-formed' u-HDA. It has recently been established that LDA and e-HDA are distinct 'phases' and that the transformation between them is discontinuous. Current focus is on the transition between e-HDA and VHDA and on the question as to whether VHDA is a distinct 'phase', but interpretation is complicated by the fact that most studies have been carried out on samples recovered to ambient pressure. I will present the results of neutron diffraction studies of the transformations, annealing behaviour and the structures of u-HDA, e-HDA and VHDA under in-situ conditions which provide new insight into the nature of e-HDA and VHDA and the transformations between them.