Revealing the Flexibility of Hydrogen Bond Networks in Hexagonal Ice by Frozen Liquid-Cell Electron Microscopy*

Ice crystals are formed by directional hydrogen bonds between water molecules and are thus considerably more flexible than typical inorganic structures based on covalent or ionic bonds. However, it is challenging to visualize such a crystalline network with atomic resolution in real space due to the weak bond strength and the resulting instability of ice under electron irradiation in vacuo. In this presentation, we report high-resolution transmission electron microscopy (HRTEM) of ice sections encapsulated between amorphous carbon membranes. Specifically, liquid cells containing water were frozen by liquid nitrogen to form hexagonal ice (Ih ice). Even though the ice section shows overall single-crystallinity by diffraction standards, the crystal exhibits structural variation and contains sub-domains sized on the 10-nm scale. Quantitative strain and tilt mapping reveals the sub-surface flexibility of ice and is used to demonstrate the correlation between defect structures and strain accumulation. We discovered that both sharp low-angle grain boundaries and gradual bending of the crystal are accountable for forming sub-domains with different crystal orientations. Taken together, this work shows promise in elucidating the molecular and defect structures in ice and may shed light on the physics of ice surfaces and sub-surfaces.