Chemistry Reference
In-Depth Information
(a)
(b)
Figure 29.
The smallest repeating unit for each of the independent lattices that generate (a)
the lowest energy and (b) second lowest energy isomer, as determined from DFT calculations on a
40-water unit cell of ice VI [160], viewed perpendicular to the
c
-axis. Structure (b) is antiferroelectric.
This arrangement of H-bonds has tetragonal symmetry and is assigned the space group
P
2
1
2
1
2
1
[163].
The H-bonds parallel to the
a
- and
b
-axes point in a counterclockwise fashion as one looks down the
c
-axis. The bonds that must be reversed to interconvert the two structures are circled in (a).
The energies, as predicted by electronic DFT (Fig. 28), can be fit using graph
invariants and a first-order transition to the
Cc
structure is found at 108 K [120].
Despite the near-degeneracy of the ferroelectric and antiferroelectric structures of
Fig. 29, fluctuations to antiferroelectric configurations do not persist to lower tem-
peratures. Simulations initialized with the H-bond configuration of Fig 29b rapidly
transformed to the ferroelectric ground state, Fig. 29a, indicating that factors be-
side energetics (i.e., entropic factors) seem to also favor the ferroelectric state.
Significant proton ordering is observed in these calculations above the transition
over a wide temperature range. If the high-temperature predictions are valid, and,
of course, there is some doubt in this case, which stems from the discrepancy with
the experimental ground state, this should be observable in calorimetric experi-
ments provided that H-bond arrangements can equilibrate on an experimental time
scale.
F.
Ice XII/XIV
A little more than a decade ago, the twelfth phase of ice was proposed based on the
results of neutron diffraction experiments. This new phase of ice was found within
the stability region of ice V by quickly cooling liquid water to 270 K at 0.55 GPa
