Fig. 3

Crystal structure of ice \(\hbox {I}_\mathrm{h}\) [61] with correlated disorder. a original unit cell with space group \({\hbox {P}{6}_3/mmc}\) (194), \(a\, = \,b\, =\, 4.497479\) Å and \(c = 7.322382\) Å \((\alpha = \beta = 90^{\circ }, \gamma = 120^{\circ }\)). 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 \(\hbox {Cmc2}_1\) (36) and c \({\hbox{Cc}}\) (9), respectively