Ice-three (ice III)
is formed from water at 300 MPa by lowering its temperature
to 250 K (see Phase Diagram).
It is stable over a relatively small range of conditions
in the P-T phase space. Its unit cell, which
forms tetragonal crystals (Space group P41212, 92; Laue class symmetry
4/mmm; analogous to keatite
silica), is shown opposite. In the crystal, all water
molecules are hydrogen bonded to four others, two as
donor and two as acceptor. Ice III contains five membered rings joined as bicyclo-heptamers
and has a density of 1.16 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. The tetragonal
crystal (shown below) is pseudo-cubic with cell dimensions
6.666 Å (a) and 6.936 Å (c; 90º, 90º,
90º; at 250 K and 280 MPa) 
and contains 12 water molecules. Its structure consists
of tight right-handeda four-fold helices, containing
two thirds of the water molecules, connected by the
remaining water molecules which, thus, experience a
differing molecular environment.
Metastable ice-three may be formed within the ice-two phase space by raising the compression rate in the density driven phase transition from hexagonal ice down to 170 K .
Ice-three has triple points with liquid water
and ice Ih (-21.985 °C, 209.9 MPa), liquid water and ice-five
(-16.986 °C, 350.1MPa), ice Ih and ice-two (-34.7 °C,
212.9 MPa) and ice-two and ice-five (-24.3 °C, 344.3 MPa).
The dielectric constant of ice-three is about 117.
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).
This disorder gives rise to a zero-point entropy
close to 3.423 J mol-1 K-1 . 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 melting curve for ice-three is given by MPa .
The ordered hydrogen-bonding form of ice III is ice IX (ice-nine).
Interactive Jmol structures are given.
a Therefore the crystals are chiral.