M. M. Koza, A. Tölle, F. Fujara (Univ. Dortmund), H. Schober, T. Hansen (ILL).
Although, water has been the object of extensive experimental
and theoretical investigation it still rewards us with new and unexpected properties.
This has been demonstrated recently by the discovery of polyamorphism [1], i.e. the existence of
two different amorphous phases of water, and by the identification of a twelfth crystalline
ice-phase (ice XII) [2].
Having been observed in different regions of water's phase diagram the two phenomena
were originally thought to be disconnected.
Ice XII is produced at about 0.55 GPa and 260 K. It provides the first example of
a four-connected network which is built up by seven- and eight-membered rings [3]. These rings are arranged
such that they lead to the densest crystalline water-phase known so far which does
not show hydrogen bond interpenetration.
Polyamorphism is experimentally confined to much lower temperatures. At T below 150
K common crystalline hexagonal ice Ih can be compressed into a high-density amorphous
state (HDA) by the application of pressure exceeding 1 Gpa. HDA can be recovered
at ambient pressure below 80 K. Upon heating HDA transforms into a low-density amorphous
phase (LDA). While the microscopic structure of HDA is still a matter of speculation
there are strong experimental indications that LDA consists of a random network of
fully connected water tetrahedra. LDA can be maintained at ambient pressure up to
T = 140 K before it crystallises into a cubic phase Ic of nearly equal density.
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Figure 1: The phase diagram of water. l represents the
region in which ice XII has been observed by Lobban et al. (T = 260 K, p = 0.55 GPa)
[2].
Please note that this region is fully surrounded by the stability range of ice V.
The inset sketches the pressure induced transition line of Ih (red line) as studied
by O. Mishima [1]. The green area indicates the region in which ice XII is successfully
formed. Horizontal arrows indicate that Ih transforms by compression below 150 K
to ice XII or high-density amorphous state (HDA) and above 150 K to ice III/IX. The
orange area displays the 150 K boundary studied by O. Mishima for HDA which is equally
observed for ice XII. |
In the past, HDA has often been found to be contaminated by crystalline phases. As
these phases could not be properly indexed in a diffraction experiment, they were
given only little attention. Our recent study, performed on the instruments D20 and
D2B at ILL, reveals that all contaminations correspond to the recently detected ice
XII modification [4,5].
This implies that ice XII forms under identical thermodynamic conditions as HDA,
i.e. at temperatures T below 150 K and at pressures p above 1 GPa. By properly choosing
the compression rate it is possible to favour the production of either ice XII or
HDA. Under rapid compression the formation of ice XII dominates, whereby compression
rates exceeding 1 GPa/min lead to pure bulk ice XII samples.
When increasing the temperature beyond 77 K, the formation of ice XII does not only
compete with HDA but also with other crystalline phases. Above 150 K no ice XII formation
has been observed in our experiments [5]. Ice XII, therefore, is rather prolific
in water's phase diagram existing in at least two topologically unconnected regions.
The study of the formation of ice Ih to ice XII contributes valuable clues to the
understanding of water amorphisation under pressure. Crystallisation implies a reorganisation
of water's hydrogen bond network and not merely its deformation. The latter is sufficient
for amorphisation. A molecular reorganisation requires a high molecular mobility
as provided by a thermodynamic mechanical instability or a melting process. The instability
or melting may be followed either by recrystallisation or amorphisation. Moreover,
since the structure of ice XII has been established as the first example of a non-self-clathrating
network built up by seven- and eight-membered rings of water molecules it may help
us to deduce some structural characteristics for HDA. It may even be possible to
model the structure of HDA by putting disorder into the molecular network of ice
XII. This process has been successfully performed on the Ih network leading to a
model structure for LDA.
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Figure 2: Transition of ice XII towards hexagonal ice upon heating. The upper left photograph a) shows ice XII as recovered from the pressure cell at low temperature. In contrast to high-density amorphous ice it has a milky appearance. The following photographs show the floking of ice XII as it transforms to the lower density forms cubic ice and hexagonal ice (b-d). |
[1] O. Mishima, Nature 384 (1996) 546. [2] C. Lobban, J. L. Finney and W. F. Kuhs, Nature 391 (1998) 268 . [3] M. O'Keeffe, Nature 392 (1998) 879. [4] M. Koza, H. Schober, A. Tölle, F. Fujara, and T. Hansen, Nature 397 (1999) 660. [5] M. Koza, H. Schober, T. Hansen, A. Tölle and F. Fujara, PRL, in print.
