Compressional rate-dependent stability of ammonia hydrates crystallized from water-rich ammonia-water solutions

TL;DR

This study explores how different compression rates influence the crystallization pathways of ammonia-water solutions at room temperature, revealing rate-dependent formation of distinct ammonia hydrate phases and ice VII.

Contribution

It demonstrates the critical role of compression rate in determining ammonia hydrate crystallization pathways using time-resolved X-ray diffraction with dynamically compressed diamond anvil cells.

Findings

High compression rates favor direct formation of bcc DMA' phase.

Lower rates stabilize monoclinic AHH-II and ice VII.

Intermediate rates produce mixed hydrate and ice phases.

Abstract

Understanding the crystallization pathways of water-rich ammonia-water (NH3-H2O) solutions and the stability of ammonia hydrates is key to unraveling the behavior of complex hydrogen-bonding networks as well as for planetary interior modelling. Yet, there are still inconsistencies in the crystallization sequence reported upon pressure-induced crystallization of H2O-rich NH3-H2O solutions at room temperature. Here, we investigate the effect of compression rates on the crystallization pathways of 25 wt% NH3 aqueous solutions at room temperature using dynamically compressed diamond anvil cells (dDAC) coupled with time-resolved X-ray diffraction. We show that compression rates exceeding 0.5 GPa/sec promote direct crystallization of a body-centered cubic (bcc) phase (DMA') with possible AMH stoichiometry coexisting with H2O ice VII, while rates below 0.2 GPa/sec stabilize monoclinic NH3-rich AHH-II and ice VII phases. Intermediate rates between 0.2-0.5 GPa/sec produce a mixture of both hydrates alongside ice VII, hence demonstrating the role of compression rate on the crystallization sequence of ammonia solutions. The compression behavior and phase stability of the distinct phase assemblies (AHH-II/DMA' + ice VII) are investigated further to place constraints on the composition of the DMA' phase, the effect of ice VII on the compressibility of ammonia hydrates, and the plausible incorporation of NH3 impurities within the lattice of high-pressure ice phases.