Ice-twelve (ice XII)
may be formed by heating high-density amorphous ice at a constant
pressure of 0.81 GPa from 77 K to ~183 K at a rate of ≥ 15
K min-1 and recovered at atmospheric pressure at
77 K ; a slower rate (≤ 0.4 K min-1) preferably producing ice-four).
Ice-twelve is metastable within the ice-five and ice-six phase space (see Phase
Diagram). It forms a tetragonal crystal (Space group , 122; Laue class symmetry 4/mmm).
In the crystal, all water
molecules are hydrogen bonded to four others, two as
donor and two as acceptor. Ice-twelve contains a screw-type
hydrogen bonded arrangement (right-handed double helix)
quite unlike that found in other crystalline forms of
ice, with the smallest ring size consisting of seven
not five as reported ;
two seven-membered rings can be seen top-left and bottom
right in the opposite sub-structure). It has a density
of 1.30 g cm-3 at 127 K and ambient pressure,
somewhat greater than ice-five (1.23 g cm-3).
The hydrogen bonding is disordered and constantly changing
as in hexagonal ice.
The tetragonal crystal
(shown opposite) has cell dimensions a, b = 8.276 Å
and c =4.027 Å (90º, 90º, 90º) and
contains 12 water molecules .
A third of these water molecules are more regularly
tetrahedral than the remainder and, thus, experience
a differing molecular environment. In the above structure,
the seven-membered rings involve water molecules above/below
each other such as found at the bottom right.
Ice-twelve is metastable within the ice-five
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
The ordered hydrogen-bonding form of ice XII is ice XIV (ice-fourteen).
Interactive Jmol structures are given.