Water's phase diagram: From the notion of thermodynamics to hydrogen-bond cooperativity

This presentation features recent progress in understanding the phase diagram of water and ice from the perspective of hydrogen bond (O:H–O) cooperative relaxation with focus on how the segmental length and the containing angle of the O:H–O bond change with mechanical compression and thermal excitation. By interplaying theoretical predictions, numerical computations, and phonon spectrometrics, we firstly examined the relaxation dynamics of O:H–O bond segmental length and phonon stiffness of: i) liquid water at 300 K and ice at 80 K as a function of pressure, ii) liquid water cooling from 350 K to 80 K under the ambient pressure, iii) mechanical freezing of the ambient water under compression up to 1.83 GPa, and, iv) liquid water heating from 253 to 753 K under 30 MPa pressure. Observations allow us to classify the TC(P) phase boundaries of water and ice into four types according to their slopes and then formulate them in terms of hydrogen bond relaxation in segmental length and containing angle. Observations reinforce the essentiality and effectiveness of hydrogen bond notion in dictating the unusual behavior of water and ice and clarify the bonding dynamics during phase transition, which is beyond the scope of classical thermodynamics.