RELATIONSHIP BETWEEN MELTING AND AMORPHIZATION OF ICE

THE discovery(1) in 1984 that an ice crystal can be transformed by pre ssure to an amorphous phase has since been followed by other examples of pressure-induced amorphization(2). This transition like melting, in volves loss of long-ranged order, prompting the question of whether th e two transitions are related. Here I describe experiments probing thi s relationship for a form of crystalline ice (denoted Ih) which is mel ted and amorphized by pressure. To avoid the complication of crystal-c rystal transformation interrupting the melting process I use an ice em ulsion, in which the very small particle size (about 5 mu m) suppresse s nucleation of other crystal phases. As the temperature is decreased, I see a smooth crossover from (pressure-induced) equilibrium melting to sluggish amorphization at around 140-165 K. In this temperature ran ge, ice Ih became 'supercompressed' before melting to a highly viscous liquid which seemed to be related to an imperfectly relaxed amorphous ice. Below about 140 K, ice Ih was transformed to an unrelaxed phase apparently related to the high-density amorphous form of ice. This seq uence of transitions can be viewed as a crossover from a two-phase mel ting process (which is determined by the relative free energies of the solid and liquid phases) towards a one-phase amorphization process (w here the transition is induced by a mechanical instability limit of th e solid).