Water structure and behavior


Ice-twelve (ice XII)


Ice-twelve (ice XII) may be formed by heating high-density amorphous ice at a constant pressure of 0.81 GPa from 77 K to ~183 K at a rate of ³15 K min-1 and recovered at atmospheric pressure at 77 K [386]; a slower rate (£0.4 K min-1) preferably producing ice-four). Ice-twelve is metastable within the ice-five and ice-six phase space (see Phase Diagram). It forms a tetragonal crystal (Space group I(-4)2d, 122; Laue class symmetry 4/mmm). In the crystal, all water molecules are hydrogen bonded to four others, two as donor and two as acceptor. Ice-twelve contains a screw-type hydrogen bonded arrangement (right-handed double helix) quite unlike that found in other crystalline forms of ice, with the smallest ring size consisting of seven molecules ([390] not five as reported [82]; two seven-membered rings can be seen top-left and bottom right in the opposite sub-structure). It has a density of 1.30 g cm-3 at 127 K and ambient pressure, somewhat greater than ice-five (1.23 g cm-3). The hydrogen bonding is disordered and constantly changing as in hexagonal ice.

[ice xii substructure]

[ice xii crystal structure]

The tetragonal crystal (shown opposite) has cell dimensions a, b = 8.276Å and c =4.027Å (90º, 90º, 90º) and contains 12 water molecules [391]. A third of these water molecules are more regularly tetrahedral than the remainder and, thus, experience a differing molecular environment. In the above structure, the seven-membered rings involve water molecules above/below each other such as found at the bottom right.

Ice-twelve is metastable within the ice-five phase space. 

Note that in this structural diagram the hydrogen bonding is ordered whereas in reality it is random (obeying the 'ice' rules: two hydrogen atoms near each oxygen, one hydrogen atom on each OO bond).

Interactive COW (Plug-in, ActiveX) and Chime structures are given.

Please submit any comments and suggestions you may have.

Water: Home | Introduction | Icosahedral water clusters | Phase Diagram | Ice-1h | Ice-1c | Ice-two | Ice-three | Ice-four | Ice-five | Ice-six | Ice-seven | Ice-eight | Ice-eleven


This page was last updated by Martin Chaplin
on 10 September, 2003

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