Structural transformations in amorphous ice and supercooled water and their relevance to the phase diagram of water

Arguably the most important liquid in our existence, water continues to attract enormous efforts to understand its underlying structure, dynamics and thermodynamics. These properties become increasingly complex and controversial as we progress into and below the ‘no man's land' where bulk water can only exist as crystalline ice. Various, so far unconfirmed, scenarios are painted for this region, including the second critical point scenario, the singularity-free scenario, and most recently the ‘critical point-free’ scenario [C.A. Angell, Science 319, 582 (2008)]. In this article the structural aspects of water in its supercooled, amorphous and confined states are explored, to the extent that these are known, and related to the water phase diagram. An important issue to emerge is the extent to which structural measurements on a disordered material can tell us about its phase.