The unusually low zero-point energy of ice II: Critical benchmarking against ice IX

Water displays a highly complex phase diagram that is key to a range of critical processes. Using ISIS facilities, we have recently shown that ammonium-fluoride doping leads to the disappearance of the hydrogen-ordered ice II in a selective fashion. This phenomenon can be explained on the basis of the topologically constraint nature of ice II, and the incompatibility of ammonium and fluoride ions with the ordered structure. Consistent with this, it has been shown computationally that ice II has an unusually low zero-point energy. Using the VESUVIO spectrometer, we could recently show that this is indeed the case. However, there is an important benchmark sample which we now need to measure: ice IX which is also hydrogen ordered and has a similar density compared to ice II. These experiments will give new exciting insights into the origins of the complexity of condensed water phases.