Structures of High and Low Density Amorphous Ice by Neutron Diffraction

TLDR

Both HDA and LDA are fully hydrogen‑bonded tetrahedral networks, with local order similar to ice Ih for LDA and to liquid water for HDA. Neutron diffraction with isotope substitution maps the structures of HDA and LDA, revealing a fifth first‑neighbor interstitial that limits first‑shell water orientations. The study shows that as one progresses from HDA through liquid water to LDA and ice Ih, the second‑shell radial order rises while spatial order falls, and the interstitial “lynch‑pin” molecule that preserves HDA structure informs the HDA‑LDA transition and the metastable water debate.

Abstract

Neutron diffraction with isotope substitution is used to determine the structures of high (HDA) and low (LDA) density amorphous ice. Both "phases" are fully hydrogen bonded, tetrahedral networks, with local order similarities between LDA and ice Ih, and HDA and liquid water. Moving from HDA, through liquid water and LDA to ice Ih, the second shell radial order increases at the expense of spatial order. This is linked to a fifth first neighbor "interstitial" that restricts the orientations of first shell waters. This "lynch pin" molecule which keeps the HDA structure intact has implications for the nature of the HDA-LDA transition that bear on the current metastable water debate.