Christoph G. Salzmann
Department of Chemistry, University College London, United Kingdom
Since the days of Bridgman and Tammann, the phase diagram of water has seen a flurry of exciting developments including the discovery of seventeen crystalline polymorphs of ice, the pressure-induced amorphisation of ice I and the existence of at least two distinct amorphous form of ice.[1] In this talk, a variety of new insights from our group into the highly complex behaviour of condensed H2O are presented. Will we ever fully understand H2O?
(1) Doping-induced disappearance of ice II from the phase diagram: Ammonium fluoride (NH4F) acts as a ‘magic ingredient’ that enables us to let ice II disappear from the phase diagram in a highly selective fashion.[2] A detailed understanding of the underlying mechanisms and thermodynamics is presented, and we argue that our new finding has wider implications that enables us to understand some of the anomalies of the phase diagram including the anomalous properties of liquid water. The selective disappearance of a phase of ice with the aid of a dopant highlights the exciting possibility of potentially discovering new phases of ice in the future using specific dopants. Furthermore, the absence of ice II also allows us to study the ice III to ice IX phase transition in great detail which was previously not accessible and we have also studied the hydrogen-disordering effect of ammonium fluoride on the ice VII to ice VIII phase transition. Finally, new experimental data on chasing the so far elusive hydrogen-disordered ice II are presented.
(2) High-density amorphous ice is a ‘derailed’ state along the ice I to ice IV pathway: The structural nature of HDA formed through low-temperature pressure-induced amorphisation of ice I is still heavily debated. We show that NH4F I, which is isostructural with ice I, undergoes a very similar pressure collapse upon compression at 77 K compared to ice. This is found for both hexagonal as well as stacking-disordered starting materials. However, the product material is not amorphous but NH4F II, a high-pressure phase isostructural with ice IV. This collapse can be rationalised in terms of a highly effective structural mechanism which we call the Engelhardt-Kamb collapse. In the case of ice I, the orientational disorder of the water molecules leads to a deviation from this mechanism and we therefore classify HDA as a ‘derailed’ state along the ice I to ice IV pathway. DFT calculations suggest that ice XI, i.e. hydrogen-ordered ice I, would indeed not undergo pressure-induced amorphisation but transform to hydrogen-ordered ice IV instead.[3]
[1] C.G. Salzmann, J. Chem. Phys. 150 (2019) 060901
[2] J.J. Shephard, B. Slater, P. Harvey, M. Hart, C.L. Bull, S.T. Bramwell, C.G. Salzmann, Nat. Phys., 14 (2018) 569–572
[3] J.J. Shephard, S. Ling, G.C. Sosso, A. Michaelides, B. Slater, C.G. Salzmann, J. Phys. Chem. Lett., 8 (2017) 1645–1650