Ice-five (ice V) is formed from liquid
water at 500 MPa by lowering its temperature to 253
K (see Phase Diagram). Its
unit cell, which forms monoclinic crystals (Space group C2/c, 15; Laue class symmetry
2/m), is shown opposite. Ice V contains four-, five-,
six- and eight-membered rings (that is, the opposite
sub-structure has two of each) and groups of seven molecules
at four different lattice sites (three consisting of
two molecules and one of a single molecule) with each
experiencing a differing molecular environment. A chain
of water molecules (O4), with each forming a corner
of two condensed four-membered rings, runs parallel
to the a-axis.
Also parallel run chains of two other
alternating sites (O2 and O3) that are joined together
through gaps in the first chain by the smaller number
of the fourth type (O1). O4 water molecules are also
hydrogen bonded to the O2 and O3 water molecules but
not to the O1 molecules. These latter molecules form
hydrogen bonds that penetrate (go through) the 8-membered
All molecules form one connected lattice
with a density of 1.24 g cm-3 (at 350 MPa
where water density = 1.13 g cm-3) .
The hydrogen bonding is disordered and constantly changing
as in hexagonal ice. This disorder gives rise to a zero-point entropy
close to 3.435 J mol-1 K-1 . The crystal has cell
dimensions a 9.22 Å, b 7.54 Å, and c 10.35 Å, (90°,
109.2°, 90°, 28 molecules; at atmospheric pressure
and 98 K) and a unit cell contains 28 water molecules
Ice-five has triple points with liquid
water and ice-three (-16.986 °C, 350.1 MPa), liquid
water and ice-six (0.16 °C, 632.4 MPa), ice-two and
ice-three (-24.3 °C, 344.3 MPa) and ice-two and ice-six
(estimated at -55 °C, 620 MPa). The relative permittivity (dielectric constant)
of ice-five is about 144.
The melting curve for ice-five is given by MPa .
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 ordered hydrogen-bonding form of ice V is ice XIII (ice-thirteen).
Interactive Jmol structures are given.