Ice XII forms on compression of hexagonal ice at 77 K via high-density amorphous water

Samples of high-density amorphous water (HDA) were prepared by compression of hexagonal ice (ice Ih) in a piston-cylinder apparatus at 77 K up to 1.8 GPa, recovered at ambient pressure under liquid N-2 and characterized by X- ray diffraction and differential scanning calorimetry. Thereafter HDA sampl es were reloaded into the piston-cylinder apparatus, compressed at 77 K up to 1.8 GPa, recovered under liquid N-2 and characterized. Formation of ice XII on compression of HDA was observed only when a sudden pronounced pressu re drop occurred at pressures greater than or similar to1.1 GPa. Pressure d rops below this threshold value, or occurring during the initial compressio n of ice Ih, did not induce formation of ice XII. Pressure-displacement cur ves obtained on compression of ice Ih are consistent with formation of ice XII from HDA, and not from ice Ih. We propose that shock waves generated by the pressure drops cause transient local heating up to the temperature ran ge of the ice V domain, and that this induces nucleation and crystal growth of ice XII. The threshold value of approximate to 1.1 GPa pressure necessa ry for the HDA --> ice XII transition indicates that the density of HDA has to reach a certain minimal value before conversion into ice XII becomes po ssible. Ice XII could thus be formed via HDA by meteorite impact on the sur faces of icy satellites of the outer planets whenever the temperatures are low.