PRESSURE-INDUCED PHASE TRANSFORMATIONS IN ICE AT LOW TEMPERATURE. J.L. Finney* & C. Lobban, Dept of Physics & Astronomy, University College London, UK, and S. Klotz & J.M. Besson, Physique des Milieux Condenses (CNRS), Universite Paris VI, France

Phase transformations on compressing D₂O ice II and ice VI at 120K to pressures in excess of 4GPa have been observed, and neutron powder diffraction data taken on the various phases using the Paris-Edinburgh high-pressure anvil cell on the POLARIS instrument at the ISIS neutron source. For the compression of ice VI, at near 4.0GPa, a rapid transformation occurred to yield a good powder of a phase whose diffraction pattern resembled ice VII, the hydrogen DISordered structure that is the equilibrium phase ABOVE 280K at this pressure. Subsequent warming above 150K resulted in a transformation to the equilibrium hydrogen-ordered ice VIII; further warming to room temperature resulted in the expected transformation to the hydrogen-disordered ice VII phase. Compressing ice II at 120K again resulted in a rapid phase change close to 4GPa; although the diffraction pattern of this phase also resembled ice VII, there was extensive structural disorder, as evidenced by broadened powder peaks (particle size estimated between 50 and 100A) and the presence of additional intensities. We tentatively identified this disordered phase with the ice VII' proposed on the basis of Raman results by Hemley et al (Nature 388 (1989) 638). On warming, this sample again transformed to ice VIII, but only at the higher temperature of about 200K, with further warming above 290K resulting in ice VII. During the initial pressure increases on both ices II and VI at 120K, there were strong indications of increased disordering, though on this instrument, we were unable to see clear evidence of the amorphous structure suggested by Hemley et al. Detailed results of the structural refinements of these phases will be presented.