NEUTRON DIFFRACTION STUDIES OF HYDROGEN BONDED ICES. J.S. Loveday#, R.J. Nelmes#, W.G. Marshall#, J.M. Besson$, S. Klotz$ and G. Hamel%. #Department of Physics and Astronomy, The University of Edinburgh, Edinburgh EH9 3JZ, U.K.; $Physique des Milieux Condensés; and $Dèpartement des Hautes Pressions, UniversitÈ P. et M. Curie, Jussieu, Paris, France
High-pressure structural studies of molecular "ices" (e.g. H2O, NH3, CH4, H2S) provide fundamental information on the behaviour of their interatomic potentials over a wide range of density. Since ices are often components of the outer planets, their high-pressure properties are also important for planetary modelling. The development of the Paris-Edinburgh cell, which has now raised the maximum pressure for neutron diffraction by a full order of magnitude to ~25 GPa, has made possible the first accurate structural studies of ices at pressures above 3 GPa.
Studies of ammonia phase IV have enabled the structure of this phase to be solved and reveal it to be orthorhombic with orientationally ordered molecules, as opposed to the rotationally disordered hexagonal-close-packed structure proposed on the basis of earlier x-ray work. Subsequent studies of D2S and methane up to ~10 GPa suggest such structural complexity is a common feature of ices under pressure. Detailed structural measurements of ices VII and VIII provide the first direct evidence of oxygen site disorder in ice VII and lead to a more detailed understanding of the precise structural relationship between these two phases. Measurements of the structural pressure dependence of ice VIII to 25 GPa reveal unexpected behaviour of the interatomic potentials as a function of increasing density and show large changes in the atomic thermal motion under pressure.