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-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%) .
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 ). 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 relative permittivity (dielectric constant)
of ice-six is the greatest of all the water ices at
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 3.504 J mol-1 K-1 . 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  and its pressure-volume data has been described [2337 ].
It was predicted that ice-six forms a hydrogen bond ordered phase
(proposed as ice-fifteen) near 108 K  or 80 K . This ice XV (ice-fifteen) phase has recently been experimentally
proven below 130 K .
Interactive Jmol structures of ice VI and ice XV are given.
a These hexamer units are similar to the lowest potential energy forms of water hexamer clusters, (H2O)6, as found using molecular simulations.