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Ice XII substructure

ice xii substructure
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 ≥ 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

ice xii crystal structure

 

 

The tetragonal crystal (shown on the left) 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 7 November, 2017


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