Ice-six (ice VI) is formed from liquid water at 1.1 GPa by lowering its temperature to 270 K. 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-hexamersa 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 with no connecting hydrogen bond, with the resulting network resembling that of packed octahedra (each tricyclo-hexamer resembling a distorted octahedron). Each network is linked together 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].

ice vi crystal structure

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]). One of the two independent networks has been given yellow hydrogen atoms.

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 Pressure=618.4+661.4x((Temperature/272.73)^4.69 -1) MPa [1320].

It was predicted that ice-six forms a hydrogen bond ordered phase (proposed as ice-fifteen) near 108 K [910] or 80 K [1003]. This ice XV (ice-fifteen) phase has recently been experimentally proven below 130 K [1582].

• Water Structure & Science: Ice VI by Martin Chaplin,

Category:Water ice