The p-T dependency of the ice II crystal structure and the effect of helium inclusion

No reliable structural data have been reported on ice II under pressure, earlier work in the literature relating either to samples recovered to ambient pressure or the helium hydrate that is formed when helium is used as the pressurizing medium. We report structural refinements of helium-free ice II at three points in the phase's region of stability. The structural differences from the helium-affected structure are significant, and can be related to the mainly repulsive interaction between the helium and both the oxygen and hydrogen atoms of the ice framework. These repulsions explain, among other changes, the different behaviors of the a (expansion) and c (contraction) lattice parameters, and the change in compressibility on the inclusion of helium. (C) 2002 American Institute of Physics.