The structure and ordering of ices III and V

The structures of ices III and V have been studied under their thermodynami c conditions of stability by neutron diffraction. The results clearly indic ate partial ordering of the water molecule orientations for both ice struct ures. For ice V the ordering is both pressure and temperature dependent, wh ile no significant changes in ordering were noted for ice III within the sm all region of stability. No reduction in symmetry, necessary for complete o rientational ordering, was observed for ice V at low temperatures. The orde ring behavior of ice V at low temperatures (< 150 K), when considered in co njunction with dielectric measurements at high temperatures, suggests that while relaxation is achieved predominantly through the diffusion of rotatio nal defects at high temperatures, the mechanism at low temperatures appears to be the migration of ionic defects which require only a small activation energy for mobilization. (C) 2000 American Institute of Physics. [S0021-96 06(00)51715-2].