Fig. 3.

Crystal structure of ice ${\text{I}}_{\mathrm{h}}$ [61] with correlated disorder. a original unit cell with space group $\text{P}{6}_3/mmc$ (194), $a=b=4.497479$ Å and $c=7.322382$ Å $(\mathit{\alpha }=\mathit{\beta }={90}^{\circ },\mathit{\gamma }={120}^{\circ }$). All O atoms (red) are on fully-occupied 4f site. H atoms have two positions: H1 4f (green) and H2 12k (gray), both with 50 % occupancy (see text). Dashed line shows unrealistic H–H distance of $\approx$0.8 Å. In b and c are two configurations satisfying the restrictions on H atoms positions that result in correlated disorder in this system, with space groups ${\text{Cmc2}}_1$ (36) and c $\text{Cc}$ (9), respectively