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 (${\mathrm{HDA}}^{\prime }$) 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 (${\mathrm{LDA}}^{\prime }$). Both HDA and ${\mathrm{HDA}}^{\prime }$ 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 ${\mathrm{LDA}}^{\prime }$, respectively. Despite their apparent structural identity on a local scale, HDA and ${\mathrm{HDA}}^{\prime }$ differ in their transition kinetics explored by in situ experiments. The activation energy of the $\mathrm{vHDA}\mathrm{\text{−}}\mathrm{\text{to}}\mathrm{\text{−}}{\mathrm{LDA}}^{\prime }$ transition is at least $20\mathrm{kJ}/\mathrm{mol}$ higher than the activation energy of the HDA-to-LDA transition.