Formation of Trilayer Ices in Graphene Nanocapillaries under High Lateral Pressure

Using molecular dynamics simulation, we investigate the phase behavior of water confined in graphene nanocapillaries at room temperature (300 K). Here, the lateral pressure Pzz is used as the primary controlling variable, and its effect on the behavior of trilayer water is systematically studied. Three (meta)stable trilayer (TL) crystalline/amorphous ice phases, namely, TL-ABAI, TL-ABA, and TL-AAAI, are observed in our simulations with the lateral pressure in the range of 1.0 GPa ≤ Pzz ≤ 6.0 GPa. TL-ABAI exhibits a square lattice in every layer, and the three layers exhibit the ABA stacking pattern; i.e., the oxygen atoms in the two outer layers are in registry. This new trilayer ice structure can also be viewed as a bilayer clathrate hydrate with water molecules in the middle layer serving as the guest molecules. With increasing lateral pressure, typically, the solid-to-liquid-to-solid phase transition occurs, during which the structural transformation from triangular to square-like in the ice layer is a...