Ice. II. A proton-ordered form of ice

This is the authors' abstract. We don't add key points for this paper.

Ice II, one of the high-pressure polymorphs of ice, of density 1.17 g.cm -3, has 12H~O molecules in a rhombohedral unit cell with a = 7-78 A, a = 113.1 . According to the Laue symmetry shown in single-crystal photographs, the space group should be either R3m, R3m or R32. Centrosymmetry is indicated by a statistical test with essentially three-dimensional intensity data. However, the Patterson function indicates a pseudo-structure in space group R3c. The true structure must be appreciably distorted from this because of moderately strong violation of the c-glide extinction condition. The distortion could degrade the space group to /~32, but the resulting structure has unsatisfactory features, and does not conform well with the experimental data. Structural reasoning indicates space group R3. The observed R3m Laue s:~nnmetry must therefore be the result of intimate twinning of Rg individuals. The structure in R3 can be refined, under the assumption of twinning, and with the inclusion of hydrogen atoms, to R = 0.08.