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Ice-three (Ice III)

Ice III unit cell

Ice III unit cell


Ice-three (ice III) is formed from water at 300 MPa by lowering its temperature to 250 K (see Phase Diagram). It is stable over a relatively small range of conditions in the P-T phase space. Its unit cell, which forms tetragonal crystals (Space group P41212, 92; Laue class symmetry 4/mmm; analogous to keatite silica), is shown on the right. In the crystal, all water molecules are hydrogen-bonded to four others, two as donor and two as acceptor. Ice III contains five-membered rings joined as bicyclo-heptamers and has a density of 1.16 g cm-3 (at 350 MPa where water density = 1.13 g cm-3). The hydrogen-bonding is disordered and continuously changing as in hexagonal ice. The tetragonal crystal (shown below) is pseudo-cubic with cell dimensions 6.666 Å (a) and 6.936 Å (c; 90º, 90º, 90º; at 250 K and 280 MPa) [385] and contains 12 water molecules. Its structure consists of tight right-handed a four-fold helices, containing two-thirds of the water molecules, connected by the remaining water molecules which, thus, experience a differing molecular environment.

Ice-three crystal

ice-three crystal

Metastable ice-three may be formed within the ice-two phase space by raising the compression rate in the density-driven phase transition from hexagonal ice down to 170 K [1545].


Ice-three has triple points with liquid water and ice Ih (-21.985 °C, 209.9 MPa), liquid water and ice-five (-16.986 °C, 350.1MPa), ice Ih and ice-two (-34.7 °C, 212.9 MPa) and ice-two and ice-five (-24.3 °C, 344.3 MPa). The relative permittivity (dielectric constant) of ice-three is about 117.


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 3.423 J mol-1 K-1 [2153]. As the H-O-H angle does not vary much from that of the isolated molecule, the hydrogen bonds are not straight (although shown so in the figures).


The melting curve for ice-three is given by    Pressure=209.5 +101.1x((Temperature/251.15)^42.86 -1)    MPa [1320].


The ordered hydrogen-bonding form of ice III is ice IX (ice-nine).


Interactive Jmol structures are given.


a   Therefore the crystals are chiral. [Back]



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This page was established in 2002 and last updated by Martin Chaplin on 20 September, 2018

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