First synthesis of an ordered form of ice Ic The University of Tokyo, Kazuki KOMATSU 1. Introduction Ice has a tremendous number of crystal forms; there are at least 18 polymorphs (Ih, Ic, II ~ XVII). In the long history of ice polymorph study, the most important and widely be influential problem could be ‘the undiscovered pure ice Ic’. Ice Ic is the ‘c’ubic form of ice with ABCABC… stacking sequence, which can be comparable to the normal ice Ih, having the ‘h’exagonal form with ABAB… sequence. Ice Ic may be commonly found in upper atmosphere in the Earth, in nano-porous materials, or in icy planet (like Pluto) or dust in universe. In spite of that many studies have carried out for ice Ic because of its distinguished importance, all studies, without any exception, have dealt with ice Ic having stacking disorder more or less. This fact could be the reason why the ordered form of ice Ic (called ice XIc) has never found so far (Salzmann et al., PCCP, 2011). The recent discovery of ice XVI (Falenty et al., Nature, 2014) gave us a hint to discover the ice XIc. The ice XVI was formed through Ne-hydrate, which has a clathrate structure – guest Ne and host ice, and evacuate Ne from the system under vacuum by using turbo-molecular pump (TMP). It is naturally considered that the similar phenomena happen for other clathrate hydrate, if the guest molecules are enough to be small. Then, we realize the hydrogen (H2) has quite small Van der Waals radius (120 pm), which is comparable to that of Ne (154 pm). The idea that hydrogen can be removable from hydrogen hydrate was very recently performed by Rosso et al., Nat. Commun. (2016) for a form of hydrogen hydrate, C0. Moreover, hydrogen hydrate formed filled ice Ic (the host ice is isostructural with ice Ic) under pressure above 2 GPa. Thus, hydrogen hydrate could be the most favored specimen for that purpose. We conducted experiments (2015A0033, 2016B0009 and 2017A0092) to make the pure ice Ic via Filled ice Ic, starting from D2O and MgD2 as a hydrogen source. These preliminary experiments are succeeded; filled ice Ic can be obtained at ambient conditions, and hydrogen was degassed under vacuum at low temperature below 100 K. Although there is a negative evidence, in a previous literature (WL Mao & HK Mao, PNAS, 101, 708 (2004)), that the filled ice Ic releases its hydrogen when downloading to ambient pressure at 77K, our results show that lower temperature than 77 K prevent filled ice Ic to decompose. Here we carried out further experiment to explore the ordered form of this pure ice Ic. 2. Experiment Powder samples of MgD2 were synthesized from MgH2 in a pressure chamber with D2 gas. The o hydrogen in MgH2 is exposed by heating up to 500 C using TMP and subsequently filled with D2 gas at a temperature of 400 oC. This temperature cycle was repeated more than 10 times to exchange hydrogen MgH2 into deuterium. The yielding powder sample was confirmed to be MgD2 structure by powder x-ray diffraction. 1 3. Results MgD2 and D2O were loaded in a sample room of a pressure-temperature variable cell (so-called ‘MITO system’), and then heated up to 403 K for 1h in order to decompose MgD2 into Mg(OD)2, D2 and D2O. The sample was compressed to ~3 GPa, after cooling down to room temperature, yielding the high pressure form of hydrogen hydrate, C2. The sample was cooled down to 100 K, and subsequently downloaded to ambient pressure. As found in the previous experiments, the pure ice Ic was reproducibly obtained at 0 GPa after decompression under 100 K. Then, the temperature was increased up to 170 K to sharpened up the peak profile. The world first structure refinement for the pure ice Ic was conducted (Fig. 1). The sample was cooled down to 50 K for 6 hours and annealed at 60 K for 1 week, to obtain the ordered form of ice Ic, but no remarkable difference was observed even after the 1week annealing. Figure 1 | Rietveld refinement for the pure ice Ic without stacking disorder with the experimental P-T path and the crystal structure of ice Ic. 4. Conclusion We managed to obtain the ice Ic and succeeded to refine the pure ice Ic without stacking disorder. However, the ordered form of ice Ic could not be observed. 2 .