Water hydrogen bonding is extremely versatile; ≈20 ice structures and several types of clathrate hydrate structures are identified. These crystalline water structures form at temperatures below room temperature and/or at high pressure. Transmission electron microscopy is used to study a new crystalline water structure in a clathrate state that is prepared by sandwiching gas-supersaturated water between graphene layers under ambient conditions. In this clathrate state, water molecules form a 3D hydrogen-bonding network that encloses gas-filled cages 2–4 nm in size. Herein, the crystalline water structure is derived by recording and analyzing electron diffraction patterns and performing first-principles calculations. The structure consists purely of pentagonal rings and has a topology similar to that of water ice XVII. In the study, a mechanism for the formation of the clathrate state is proposed. The results improve the understanding of interactions among water and small nonpolar molecules and offer insights into the local structures of ambient liquid water.
