Mechanism and kinetics of the reversible transformation lda-hda of amorphous ice under pressure

An ultrasonic investigation of pressure-induced direct and reverse transfor mations lda-hda between the amorphous phases of low- and high-density ice i s performed and the change in the shear modulus is determined. It is found that elastic softening of the amorphous network precedes both transformatio ns, and the behavior of lda phase is similar to that of hexagonal 1h ice be fore amorphization. At the same time a number of features of the lda-hda tr ansformation are due to the topological disordering of the amorphous phases , and the kinetics of the transformation is different from the standard beh avior at a first-order phase transition and also from the compression kinet ics of "classic" glasses of the type a-SiO2 and a-GeO2. It is shown that th e difference in the behavior of the lda and hda amorphous phases under pres sure can be attributed to characteristic features of their spectra of low-e nergy two-level states. (C) 1999 American Institute of Physics. [S0021-3640 (99)00909-3].