Observation of ammonia dihydrate in the AMH-VI structure at room temperature – possible implications for the outer solar system

Abstract

Ammonia dihydrate (ADH) is an important constituent of the outer solar system and its high-pressure behaviour is relevant to the modelling of Titan, Uranus and Neptune. Our neutron diffraction studies show that ADH can exist at room temperature in the substitutionally disordered structure of the ammonia monohydrate (AMH) phase VI. This implies that a solid solution may exist between ADH and AMH at high pressure, and this is of probable importance to models of the outer solar system.

Keywords:

• neutron diffraction
• simple molecular systems
• icy moons

Acknowledgements

We would like to acknowledge the technical assistance of Mr D.J. Francis of the ISIS Facility, and the helpful advice of two anonymous referees. The work was supported by the Engineering and Physical Science Research Council and the Science and Technology Facilities Council.

Notes

We note that Fortes et al. 8 state that the number of hydrogen atoms in their fits was that of the ADH composition, but the number of hydrogen atoms in their unit cell (seven) is incorrect and there are actually 4.667 hydrogen atoms in an average unit cell of bcc-ADH. Nevertheless, it is clear that the diffraction pattern they obtained is that of the AMH-VI structure.

Additional information

Notes on contributors

M. Guthrie

† Present address: Advanced Photon Source, Argonne National Laboratory, South Cass Avenue, Argonne, IL, USA.