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Title: On the stability of the disordered molecular alloy phase of ammonia hemihydrate

The disordered-molecular-alloy phase (DMA) of ammonia hydrates [J. S. Loveday and R. J. Nelmes, Phys. Rev. Lett. 83, 4329 (1999)] is unique in that it has substitutional disorder of ammonia and water over the molecular sites of a body centred cubic lattice. Whilst this structure has been observed in ammonia di- and mono-hydrate compositions, it has not been conclusively observed in the ammonia hemihydrate system. This work presents investigations of the structural behaviour of ammonia hemihydrate as a function of P and T. The indications of earlier studies [Ma et al. RSC Adv. 2, 4290 (2012)] that the DMA structure could be produced by compression of ammonia hemihydrate above 20 GPa at ambient temperature are confirmed. In addition, the DMA structure was found to form reversibly both from the melt, and on warming of ammonia hemihydrate phase-II, in the pressure range between 4 and 8 GPa. The route used to make the DMA structure from ammonia mono- and di-hydrates—compression at 170 K to 6 GPa followed by warming to ambient temperature—was found not to produce the DMA structure for ammonia hemihydrate. These results provide the first strong evidence that DMA is a thermodynamically stable form. A high-pressure phase diagram formore » ammonia hemihydrate is proposed which has importance for planetary modelling.« less
Authors:
 [1] ;  [2] ;  [3] ;  [2] ; ; ; ;  [3]
  1. Diamond Light Source, Ltd., Harwell, Didcot, Oxfordshire OX11 0DE (United Kingdom)
  2. (United Kingdom)
  3. SUPA, School of Physics and Astronomy, Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3JZ (United Kingdom)
Publication Date:
OSTI Identifier:
22416231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 142; Journal Issue: 9; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; AMBIENT TEMPERATURE; AMMONIA; AMMONIUM COMPOUNDS; BCC LATTICES; COMPRESSION; HYDRATES; PHASE DIAGRAMS; PHASE STABILITY; PRESSURE DEPENDENCE; TEMPERATURE DEPENDENCE; WATER