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ice xii substructureIce-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 ≥ 15 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 bar4 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 crystal structure



The tetragonal crystal (shown opposite) has cell dimensions a, b = 8.276 Å and c =4.027 Å (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 O····O bond). This disorder gives rise to a zero-point entropy close to 3.393 J mol-1 K-1 [2153].


The ordered hydrogen-bonding form of ice XII is ice XIV (ice-fourteen).


There is a reported triple point between the metastable phases of Ice XII and Ice IV and liquid water at -6 °C and ~500-600 MPa [1300], but this needs confirmation.


Interactive Jmol structures are given.



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This page was established in 2002 and last updated by Martin Chaplin on 18 February, 2016

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