(ice VII) and ice ten (ice X)
(ice VII) is formed from liquid water above 3 GPa by lowering its
temperature to ambient temperatures (see Phase
Diagram). Its unit cell, which forms cubic crystal (Pn3m, 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 ),
which is less than twice the cubic ice density as the intra-network
O····O distances are longer 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.
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.
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
(81.6°C, 2.2 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
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).
COW (Plug-in, ActiveX)
and Chime (35 KB) structures are given.
the pressure is raised, ice-seven appears to undergo a continuous
transition into cubic ice-ten (ice X) where the ice protons
are equispaced between the oxygen atoms .
The oxygen atoms are arranged in a body-centered cubic arrangement
(8 neighbors) and the hydrogen atoms in a body-centered
truncated cubic arrangement (12 neighbors).
and Chime (48 KB) structures are