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 important for understanding not only the complex condensed states of H2O 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 (NH4F), 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 NH4F 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.
Highlights
A simple compression experiment of the ‘ordinary’ ice Ih at liquid-nitrogen temperature has left a remarkable legacy and tremendous impact across the materials sciences.[1]. It marked the discovery of high-density amorphous ice (HDA) which provided the first example for the process of pressure-induced amorphization (PIA)
The PIA of ice Ih was first rationalized in terms of a thermodynamic melting process implying that HDA is a glassy state of high-pressure water.[1, 8,9,10,11,12]
Due to the pivotal position of HDA for understanding the condensed states of H2O and in the wider context of PIA processes, a deeper knowledge of the mechanism of the PIA of ice I is of paramount importance
Summary
A simple compression experiment of the ‘ordinary’ ice Ih at liquid-nitrogen temperature has left a remarkable legacy and tremendous impact across the materials sciences.[1]. We first investigate the low-temperature compression behavior of hexagonal and stacking-disordered ammonium fluoride (NH4F) whose hydrogen-bonded networks are similar compared to the corresponding members of the ice I family.
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