Two-Dimensional Water and Ice Layers:  Neutron Diffraction Studies at 278, 263, and 20 K

Neutron diffraction elucidates the structures of two-dimensional (2D) water layers (278 K) or 2D ice layers confined in an organic slit-shaped nanospace. The two-dimensional ice phases reported here consist of individual eight-membered rings or folded-chain segments (263 K) and condensed twelve-membered irregular rings (20 K). This is quite different from bulk or other 2D ice structures; the latter usually form hexagonal honeycomb lattices. Both low-temperature structures typically feature water molecules which are surrounded by two or three other water molecules. Neutron diffraction and thermochemical studies indicate a liquid-solid-phase transition around 277 K for two-dimensional D2O layers. A further solid-solid-phase transition occurs between 263 and 20 K.