Structural differences between unannealed and expanded high-density amorphous ice based on isotope substitution neutron diffraction

Katrin Amann-Winkel,Daniel T Bowron,Thomas Loerting

doi:10.1080/00268976.2019.1649487

Abstract

We here report isotope substitution neutron diffraction experiments on two variants of high-density amorphous ice (HDA): its unannealed form prepared via pressure-induced amorphization of hexagonal ice at 77 K, and its expanded form prepared via decompression of very-high density amorphous ice at 140 K. The latter is about 17 K more stable thermally, so that it can be heated beyond its glass-to-liquid transition to the ultraviscous liquid form at ambient pressure. The structural origin for this large thermal difference and the possibility to reach the deeply supercooled liquid state has not yet been understood. Here we reveal that the origin for this difference is found in the intermediate range structure, beyond about 3.6 Å. The hydration shell markedly differs at about 6 Å. The local order, by contrast, including the first as well as the interstitial space between first and second shell is very similar for both. ‘eHDA’ that is decompressed to 0.20 GPa instead of 0.07 GPa is here revealed to be rather far away from well-relaxed eHDA. Instead it turns out to be roughly halfway between VHDA and eHDA – stressing the importance for decompressing VHDA to at least 0.10 GPa to make an eHDA sample of good quality.

Highlights

High-density amorphous ice (HDA) was discovered by Mishima et al in the 1980s [1]
Decompression to 0.07 GPa yields the most relaxed expanded HDA (eHDA) state that directly converts to low-density amorphous ice (LDA) in a first-order like transition. It is unclear how well relaxed eHDA states are that are decompressed to higher pressures, and so we investigate this question based on isotope substitution neutron diffraction
We study the historically most studied form as prepared following the protocol of pressure-induced amorphization of ice Ih at 77 K [30]. This so-called unannealed high-density amorphous ice (HDA) is compared with expanded forms of HDA, namely two different types of eHDA – one that is decompressed to 0.20 GPa and one that is decompressed to 0.07 GPa at 140 K

Summary

Introduction

High-density amorphous ice (HDA) was discovered by Mishima et al in the 1980s [1]. They coined the concept of polyamorphism by demonstrating that HDA represents a second form of amorphous ice, distinct from low-density amorphous ice (LDA) that has long been known [2]. The study of the site-site radial distribution functions (RDFs) in amorphous ices has traditionally been done using the technique of isotope-substitution neutron diffraction. These studies were pioneered in collaboration with Alan Soper in the early 2000s. Its microscopic structure was again reported in collaboration with Alan Soper [7] These three amorphous ices all share the same basic building block – the Walrafen pentamer.

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