Ice-seven (ice VII)
 is formed from liquid water above 3 GPa by lowering its temperature
to ambient temperatures (see Phase Diagram).
It can be obtained at low temperature and ambient pressure
by decompressing (D2O) ice-six below 95 K and is metastable over a wide range of pressure,
transforming into LDA above 120 K .
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 Ice VII unit cell, which forms cubic crystal
(, 224; Laue class symmetry m-3m) consists of two interpenetrating cubic
ice lattices with hydrogen bonds passing through
the center of the water hexamers and no connecting hydrogen-bonds
between lattices. It has a density of about 1.65 g cm-3 (at 2.5 GPa and 25 °C ),
which is less than twice the cubic ice density as the
intra-network O····O distances
are 8% longer (at 0.1 MPa) to allow for the interpenetration.
The cubic crystal (shown opposite) has cell dimensions
3.3501 Å (a, b, c, 90º, 90º, 90º;
D2O, at 2.6 GPa and 22 °C )
and contains two water molecules.
All molecules experience identical molecular
environments. The hydrogen bonding is disordered and
constantly changing as in hexagonal ice but ice-seven
undergoes a proton disorder-order transition to ice-eight at about 5 °C; ice-seven and ice-eight having identical
structures apart from the proton ordering. Ice-seven
is metastable indefinitely at 77 K.
If the pressure is increased above about 14 GPa There are changes in the proton ordering . This is shown in the phase diagram and gives rise to a loss of the cubic symmetry. At higher pressures, ice-seven undergo a
continuous transition into cubic ice-ten (ice X) where the hydrogen atoms are situated midway between the oxygen atoms. The size of the ice crystal (two molar volumes per unit cell) continuously reduces over this large change in pressure with no obvious volume steps at the expected ice transitions .
Ice-seven has known triple points with ice-six
and ice-eight (5 °C, 2.1 GPa), ice-eight and ice-ten (100
K, 62 GPa) and liquid water and ice-six (355 K, 2.216 GPa).
Interestingly, at high pressures (~ 2.3 GPa), liquid water
can be made to freeze at over 100 °C (to give the more
dense ice-seven). The dielectric constant of ice-seven is
In contrast to ice Ih,
high density ices may incorporate some solutes into their
crystal structure. Ice-seven may incorporate up to ~7.5 wt
% NaCl into the face centered positions of its body centered
cubic structure at high pressure (for example, 4-21 GPa)
this incorporation causes a reduction in the unit cell dimensions
and a greater than expected increase in density .
Interactive Jmol structures are given.