Ferroaxial Order of the Monolayer Ice in Martyite

Ice Ih, the most stable phase of water at ambient pressure, is a stacking of the honeycomb network of water molecules H₂O. What if one layer of ice is exfoliated and confined to a two-dimensional (2D) sheet? Martyite Zn₃(V₂O₇)(OH)₂·2H₂O, a mineral with the honeycomb lattice of H₂O in the porous framework, is an ideal system for studying such monolayer ice. Due to the geometrical frustration and 2D nature, H₂O molecules are dynamically disordered at room temperature. In this study, we reveal disorder-order transitions of H₂O in martyite using single-crystal X-ray diffraction (XRD). The XRD results visualize the formation of hydrogen-bonded toroidal H₂O hexamers, leading to the ferroaxial order below 200 K. Combined with the molecular dynamics simulations, we discuss the formation process of the H₂O hexamers and how they compromise the molecular arrangement toward lower temperatures. Our results demonstrate that martyite is a model system for studying 2D ice, which may give a clue for understanding the polymorphism of H₂O.