Structural disorder in the ices

For the first time an in situ neutron single crystal diffraction study on one of the high pressure ices has been performed. Crystals of heavy ice VI have been grown from the liquid inside the pressure cell at ~0.9 Gpa/9 (kbar). The new cell allows a largely unrestricted three-dimensional data collection and has been mounted on the D9 offset Eulerian cradle. High resolution data of a quality comparable to ambient pressure data have been obtained. Details of the molecular disorder were studied. The oxygen atoms are found displaced in a direction opposite to the molecular bisectrix as seen in the figure below.

click on figure for more details

The resulting intramolecular geometry is quite different from the previously established values. This is conformation that the geometry of the water molecule is by far more "normal" (i.e. closer to the free water molecule) than indicated by earlier low-resolution crystallographic work. The small, but highly significant increase of the OH(D) bond-length by some 0.02Ĺ as found in recent high-resolution studies must be ascribed to the cooperative of the water-water interactions.

Two further experiments on the ambient pressure phase of ice have been performed.

The first was dedicated to study and compare the structures of normal and heavy ice Ih at extreme resolution. Using the two-stage displex cryorefrigerator, installed on the D9 offset Eulerian cradle, data was collected at 15K out to sin theta/lambda-values of 1.6Ĺą. Structural quantum effects have been detected unequivocally for the first time in one of the phases of the solid water. The disorder deformation densities shown below show a significant difference between light and heavy ice Ih.
click on figure for more details

This difference must be due to a wider range of hydrogen bond distances (and angles) manifesting a more pronounced molecular disorder in H2O ice Ih.
Finally in a study of heavy ice Ih two degrees below the melting point, data of excellent quality have been obtained on D9 proving the reliability of the sample environment. As expected, the molecular disorder increases non-linearly on approaching the melting point. It is planned to compare the observed atomic probability densities with the results from molecular dynamic simulations in order to test the water-water interaction potentials used in the latter. A first comparison indicates that real ice Ih near the melting point is more "crystalline" (i.e. shows less orientation disorder) than predicted by recent simulations of ice melting.

Last updated by Andrew Crowe on 07/02/1996