Ice-seven (ice VII) [1226] 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 [948].
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). Its unit cell, which forms cubic crystal
(
, 224; Laue class symmetry m-3m). Ice VII 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 [8]),
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 [361])
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 [1428]. 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.
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 about 150.
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) [955]. Counter-intuitively, this incorporation causes a reduction in the unit cell dimensions and a greater than expected increase in density [955].
Interactive Chime (35 KB) structures are given.
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This page was last updated by Martin Chaplin on 26 July, 2011