Ice-six (ice VI) is formed from liquid water at 1.1 GPa by lowering its temperature to 270 K (see Phase Diagram). Its unit cell, which forms tetragonal crystals (P42/nmc, 137; Laue class symmetry 4/mmm) is analogous to edingtonite silica. In the crystal, all water molecules are hydrogen bonded to four others, two as donor and two as acceptor.
Ice VI contains four membered rings joined as tricyclo-hexamers and has a density of 1.31 g cm-3 (at 0.6 GPa where water density = 1.18 g cm-3; the difference in density is only 0.03 g cm-3 at 81.6°C and 2.15 GPa). There are two separate interpenetrating networks, linked through the four equatorial water molecules in the hexamers. The two axial hexamer water molecules join the hexamers and experience a different molecular environment. The hydrogen bonding is disordered and constantly changing as in hexagonal ice. There are two distinct types of water molecule (labeled a (20%)and b (80%) and four distinct types of hydrogen bond labeled 1 (20%), 2 (40%), 3 (20%), and 4 (20%) [1003].
The tetragonal crystal (shown opposite) has cell dimensions a, b = 6.1812 Å, c = 5.698 Å (90º, 90º, 90º; D2O, at 1.1 GPa and 225 K) and contains 10 water molecules [360]).
Ice-six has triple points with ice-two and ice-five (estimated at -55°C, 620 MPa), liquid water and ice-five (-0.16°C, 632.4 MPa), ice-seven and ice-eight (~5°C, 2.1 GPa) and liquid water and ice-seven (355 K, 2.216 GPa). The dielectric constant of ice-six is the greatest of all the water ices at about 193.
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). 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-six is given by 
MPa [1320].
It is predicted, but not yet experimentally proven, that ice-six forms a hydrogen bond ordered phase (proposed as ice-fifteen) near 108 K [910] or 80 K [1003].
The ordered hydrogen-bonding form of ice VI is ice XV (ice-fifteen).
Interactive Chime (28 KB) structures are given.
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This page was last updated by Martin Chaplin on 23 June, 2008
