Is High-density Amorphous Ice Simply a 'Derailed' State along the Ice I to Ice IV Pathway?

TL;DR

This paper investigates whether high-density amorphous ice (HDA) is a 'derailed' state in the ice I to ice IV transition, using pressure experiments and comparisons with ammonium fluoride to understand its structural nature.

Contribution

It introduces the idea that HDA is a 'derailed' state along the ice I to ice IV pathway, based on pressure-induced transformations and comparisons with similar hydrogen-bonded systems.

Findings

HDA is classified as a 'derailed' state along the ice I to ice IV pathway.

Ammonium fluoride undergoes a pressure collapse to a phase isostructural with ice IV.

The orientational disorder of water molecules in ice I causes deviation from the ideal transition mechanism.

Abstract

The structural nature of high-density amorphous ice (HDA), which forms through low-temperature pressure-induced amorphization of the 'ordinary' ice I, is heavily debated. Clarifying this question is not only important for understanding the complex condensed states of H$_2$O but also in the wider context of pressure-induced amorphization processes, which are encountered across the entire materials spectrum. We first show that ammonium fluoride (NH$_4$F), which has a similar hydrogen-bonded network to ice I, also undergoes a pressure collapse upon compression at 77 K. However, the product material is not amorphous but NH$_4$F II, a high-pressure phase isostructural with ice IV. This collapse can be rationalized in terms of a highly effective mechanism. In the case of ice I, the orientational disorder of the water molecules leads to a deviation from this mechanism and we therefore classify HDA as a 'derailed' state along the ice I to ice IV pathway.