Chemistry Reference
In-Depth Information
Figure 22.
Third-dimensional
probability distribution of the posi-
tion of the oxygen centers relative to
the perfect lattice site, small sphere,
obtained from Monte Carlo simula-
tions at 300 K. The center of each lobe
is located on one of the
100
axes.
[141] indicated that a progressive ordering takes place as the temperature is lowered
from 210 to 165 K. Ice III, when cooled at about 1 K min
−
1
or faster, transforms to
a metastable H-bond ordered version known as ice IX, that has the same symmetry
as its proton-disordered counterpart [45, 134, 138, 141, 142]. Subsequent warming
of ice IX back across the transition results in the formation of ice II, the stable
phase in this region of the phase diagram. The space group
P
4
1
2
1
2 allows for
the possibility that the H-bonds may be partially ordered or disordered in ice III
and IX, respectively. La Placa et al. [134] identified one of four possible H-bond
arrangements possessing the appropriate symmetry, configuration (d) in Fig. 23,
to be the structure of ice IX, in agreement with earlier suggestions [138, 141].
The error in their refinement was acceptable only after deuterons were allowed to
fractionally occupy sites other than those of the dominant H-bond arrangement.
They proposed that this disagreement with dielectric experiments [141], which
suggested a fully ordered structure, may have resulted either from the rate of
cooling or as a feature of the ice IX structure. Calculations reported by Handa
et al. [143] also suggested that ice IX was fully ordered, but their calculated heat
of transition was almost twice that measured by Nishibata and Whalley [142]. This
discrepancy has yet to be resolved.
Using a differential calorimetric technique, Nishibata and Whalley [142] mea-
sured the heat of transition,
56 cal mol
−
1
, for the ice III-IX phase transition. The
−
0
.
32 cal K
−
1
mol
−
1
, is nearly 40% of the expected
measured change in entropy,
−
