Mechanisms for pressure- and time-dependent amorphization of ice under pressure

Amorphization of hexagonal ice under pressure at low temperatures has been studied by measuring the thermal conductivity with time, by keeping the ice in the temperature range 127--1300.3em0exK and pressure range 0.79--0.880.3em0exGPa. The results have been used to examine the relative merits of the Lindemann melting and the Born instability as a mechanism for amorphization of ice. Thermal conductivity decreases continuously with time according to a stretched-exponential relation. This and the findings that, (i) amorphization pressure is lower than the available values calculated for an ideal ice crystal by using the Born theory, and (ii) amorphization of cubic ice occurs at about the same pressure as that of hexagonal ice, seem inconsistent with both mechanisms. The findings can be reconciled with the Born mechanism if the effect of the existing lattice faults and/or the lattice faults produced during the plastic deformation of uniaxially compressed ice are considered to produce a distribution of collapse pressure. It is argued that the difference between the amorphization and the low-density amorph to high-density amorph transformation pressures corresponds to the energy needed to collapse, or randomly distort, the ice crystallites to a structure whose energy spontaneously decreases by diffusion of a H₂O molecule into the first coordination shell of the amorph's known structure, and persist there as an interstitial molecule. The newly observed amorphization kinetics has a bearing on the computer-simulated inference on pressure-induced collapse and/or melting of crystals.