A new form of H2O ice similar to SiO2

Figure caption: The structure of (H<sub>2</sub>O)<sub>2</sub>H<sub>2</sub> with three formula units per unit cell. The hexagonal structure is chiral, consisting of interpenetrating spiral chains of hydrogen-bonded water molecules and rotationally disordered H<sub>2</sub> molecules, and shows topological similarities with the mineral quartz.
Figure caption: The structure of (H2O)2H2 with three formula units per unit cell. The hexagonal structure is chiral, consisting of interpenetrating spiral chains of hydrogen-bonded water molecules and rotationally disordered H2 molecules, and shows topological similarities with the mineral quartz.

Strong parallels exist between the packing of tetrahedra and network topologies found in H2O and SiO2 phases. For examples, tridymite and cristobalite are analogues of ice Ih and Ic, respectively; low-energy hypothetical ice analogues of quartz have been predicted theoretically. By loading binary H2 + H2O samples in diamond anvil cells, a team used HPCAT facility and discovered a unique hydrogen-filled, ice-based chiral structure with oxygen topology very similar to the mineral quartz and other, as-of-yet-hypothetical, three-dimensional nets. Structural examination by single crystal x-ray diffraction shows that the H-bonded network forms large channels along the crystallographic c-axis, leaving the 6a Wyckoff position available for rotationally disordered H2 molecules, which is fully consistent with the single-site, filled-ice-like structure inferred from Raman measurements. Based on the structural model, the nominal composition of the C0 phase is (H2O)2H2 (three formula units per cell) assuming half occupancy of H2 positions. Details in Strobel et al, J. Am. Chem. Soc. 138, 13786, 2016.