Ice-four (ice IV)
may be formed by the heating high-density amorphous
ice at a slow (0.4 K min-1) rate from 145
K and at a constant pressure of 0.81 GPa 
(a faster rate, for example, 15 K min-1,
preferably produces ice-twelve).
Ice-four is metastable within the ice-three, ice-five and ice-six phase space (see Phase Diagram).
It forms a rhombohedral crystal (Space group , 167; Laue class symmetry -3m1) with
cell dimensions 7.60 Å (a, b, c; 70.1°, 70.1°,
70.1°, 16 molecules) .
In the crystal, all water molecules are hydrogen bonded
to four others, two as donor and two as acceptor. The
structure is formed from two interpenetrating networks
of both puckered and flattish hexamers (these allow
the inter-penetration) consisting of more strongly hydrogen
bound water. a
The penetrating hydrogen bond is longer (2.921 Å)
but these water molecules also possess three shorter hydrogen
bonds (2.783 Å). The hydrogen bonds forming the
flattish rings are also somewhat extended (2.876 Å).
The networks are not entirely independent as three quarters
of the water molecules have one weaker hydrogen bond to
the other network (2.806 Å, 143°); the fourth
type of hydrogen bond present. Hence, molecules fall into
two unequal classes experiencing different molecular environments.
Note that in this structural diagram the
hydrogen bonding is ordered whereas in reality it is
(obeying the 'ice rules': two hydrogen atoms near each
oxygen, one hydrogen atom on each O····O
bond). Ice-four is one of only two disordered ice crystals (with cubic ice) where the ordered form has not yet been found.
Interactive Jmol structures are given.
a The hydrogen-bonded
structure of ice-four has been questioned 
but recent Raman spectra seems to support the structure presented
here . It may be that it
is more stable as the deuterated ice. [Back]