The ammonia hydrates—model mixed-hydrogen-bonded systems

Abstract

The three ammonia hydrates (ammonia dihydrate, ADH; ammonia monohydrate, AMH and ammonia hemihydrate, AHH) are amongst the simplest systems to contain mixed nitrogen oxygen–hydrogen bonds. In addition, ADH and AMH are believed to be common in the outer solar system, and their properties are important for modelling bodies such as Uranus, Neptune and Titan. Neutron diffraction studies have provided the first full information on structures at ambient pressure with detailed information on the H-bond geometry and state of hydrogen ordering. High-pressure studies of AMH reveal a rich and complex high-pressure phase diagram. This work reveals that ammonia hydrates are likely to provide a wide variety of geometries for the study of mixed H-bonds. It also provides new experimental information on which to base modelling of Titan.

Keywords:

• Ammonia hydrates
• Neutron diffraction
• Hydrogen bonding
• Planetary modelling

Acknowledgements

We acknowledge the assistance of D. J. Francis and M. Guthrie in the experiments, S. Klotz and G. Hamel for technical advice, and J. Tomkinson and M. Nicol for advice on sample preparation and discussions of the results. This work is supported by grants from the Engineering and Physical Sciences Research Council and by CCLRC through provision of ISIS beamtime and other resources. This work was inspired by our interaction with Jean-Michel Besson, and we dedicate this paper to his memory.