Ice-eight (ice VIII)
is formed from ice-seven (ice VII)
by lowering its temperature (see Phase
Diagram).The hydrogen bonding is ordered and fixed
as ice-seven undergoes a proton disorder-order transition
to ice eight when cooled at about 5 °C; ice-seven
and ice-eight having identical structures apart from
the proton ordering. The proton ordering causes a slightly
distortion in the ice-seven cubic lattice (a,b slighltly
shorter, c slightly longer) resulting in a tetragonal
crystal structure (I41/amd, 141;
Laue class symmetry 4/mmm) where all of the water molecules
are hydrogen bonded to four others, two as donor and
two as acceptor. Similarly to ice-seven, ice-eight consists
of two interpenetrating cubic ice lattices.
It has a density of about 1.66 g cm-3 (at 8.2 GPa and 223 K ),
which is less than twice the cubic ice density as the
intra-network O····O distances
are longer to allow for the interpenetration. Ice-eight's
molar volume is slightly smaller (by 0.65 mm3 mol-1) than that of ice-seven along the phase
transition line. Ice-eight has triple points with ice-six
and ice-seven (5 °C, 2.1 GPa) and ice-seven and ice-ten
(100 K, 62 GPa). The dielectric constant of ice-eight
is about 4.
The crystal (shown opposite) has cell
dimensions a, b = 4.4493 Å, c = 6.413 Å
(90º, 90º, 90º; D2O, at 2.6
GPa and 22 °C ),
containing eight water molecules per unit cell. All
molecules experience identical molecular environments.
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).
Interactive Jmol structures are given.