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Updated: May 24, 2026

Methane Hydrate Crystallization on Sessile Water Droplets
Published on: May 26, 2021
Pressure-induced dehydration and the structure of ammonia hemihydrate-II
C W Wilson1, C L Bull, G Stinton
1SUPA, School of Physics and Astronomy, Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom. c.wilson-10@sms.ed.ac.uk
Abstract:
The structure of the crystalline ammonia-bearing phase formed when ammonia monohydrate liquid is compressed to 3.5(1) GPa at ambient temperature has been solved from a combination of synchrotron x-ray single-crystal and neutron powder-diffraction studies. The solution reveals that rather than having the ammonia monohydrate (AMH) composition as had been previously thought, the structure has an ammonia hemihydrate composition. The structure is monoclinic with spacegroup P2(1)/c and lattice parameters a = 3.3584(5) Å, b = 9.215(1) Å, c = 8.933(1) Å and β = 94.331(8)° at 3.5(1) GPa. The atomic arrangement has a crowned hexagonal arrangement and is a layered structure with long N-D···N hydrogen bonds linking the layers. The existence of pressure-induced dehydration of AMH may have important consequences for the behaviour and differentiation of icy planets and satellites.
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