Nature of Amorphous Polymorphism of Water

We report elastic and inelastic neutron scattering experiments on different amorphous ice modifications. It is shown that an amorphous structure (HDA^') indiscernible from the high-density phase (HDA), obtained by compression of crystalline ice, can be formed from the very high-density phase (vHDA) as an intermediate stage of the transition of vHDA into its low-density modification (LDA^'). Both HDA and HDA^' exhibit comparable small-angle scattering signals characterizing them as structures heterogeneous on a length scale of a few nanometers. The homogeneous structures are the initial and final transition stages vHDA and LDA^', respectively. Despite their apparent structural identity on a local scale, HDA and HDA^' differ in their transition kinetics explored by in situ experiments. The activation energy of the vHDA-to-LDA^' transition is at least 20 kJ/mol higher than the activation energy of the HDA-to-LDA transition.