On the nature of amorphous polymorphism of water

TL;DR

This study investigates the amorphous forms of water using neutron scattering, revealing structural similarities and differences in transition kinetics between various amorphous ice modifications.

Contribution

It demonstrates that HDA' is structurally similar to HDA but differs in transition kinetics, providing new insights into amorphous water polymorphism.

Findings

HDA' is structurally indistinguishable from HDA but differs kinetically.

HDA and HDA' show nano-scale heterogeneity in small angle scattering.

Activation energy for vHDA to LDA' transition is significantly higher than for HDA to LDA.

Abstract

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 nano-meters. 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.