EXPERIMENTAL CONFIRMATION OF THE INSTABILITY OF THE CRYSTAL-STRUCTUREOF IH ICE PRIOR TO AMORPHIZATION UNDER PRESSURE

The pressure dependences of the propagation velocities of transverse u ltrasonic waves were determined for polycrystalline ice Ih and high-de nsity amorphous ice (hda) in the pressure range 0-1.7 GPa at 78 K. It was determined that the velocity of ultrasound decreases significantly (similar to 10%) in Ih ice polycrystal before amorphization (T=78 K, p=1.1 GPa) and, correspondingly, the shear modulus softens, which is a n experimental indication of mechanical instability of the crystal lat tice occurring as a result of solid-phase amorphization under pressure . When the amorphous ice (hda) is heated from 78 K at a pressure p sim ilar to 0.05 GPa, anomalous behavior of the transverse velocity of ult rasound is observed at temperatures of 100-130 K. This behavior is evi dently associated with a phase transition of hda into amorphous low-de nsity ice (Ida). (C) 1995 American Institute of Physics.