Abstract
Ice clathrate of type II structure containing tetrahydrofuran molecules, when compressed to ∼ 15 kbar at 77 K, undergoes an irreversible transformation to a phase with different dielectric properties, but without an appreciable densification. A transformation similar to hexagonal ice does not occur at the expected melting pressure of ∼ 3·7 kbar. This suggests that the collapse of crystalline order and of the hydrogen-bonded structure under a uniaxial stress is a more likely cause of the pressure-induced amorphization than is melting along the extrapolated phase boundary. The permittivity of ice clathrate decreases on phase transformation, while that of hexagonal ice increases. These are related to a decrease in the orientation polarization of the tetrahydrofuran molecules in the first case, and of the frequency of vibrational modes that contribute to the infrared polarization in the second case. The latter indicates an increase in the average near-neighbour oxygen-oxygen distance on phase transformation in hexagonal ice.