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arXiv · 1701.05398

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

Jacob J. Shephard·Sanliang Ling·Gabriele C. Sosso·Angelos Michaelides·Ben Slater·Christoph G. Salzmann
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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.

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Jacob J. Shephard, Sanliang Ling, Gabriele C. Sosso, Angelos Michaelides, Ben Slater, Christoph G. Salzmann. 2017-01-19. Is High-density Amorphous Ice Simply a 'Derailed' State along the Ice I to Ice IV Pathway?. https://arxiv.org/abs/1701.05398

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