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

DOI: 10.1063/1.4913684 DOI Help
PMID: 25747099 PMID Help

Authors: Craig Wilson (Diamond Light Source) , C. L. Bull (University of Edinburgh) , Graham Stinton (University of Edinburgh) , Daniel Amos (University of Edinburgh) , Mary-ellen Donnelly (University of Edinburgh) , John Loveday (University of Edinburgh)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: The Journal Of Chemical Physics , VOL 142 (9) , PAGES 56

State: Published (Approved)
Published: March 2015

Abstract: 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 for ammonia hemihydrate is proposed which has importance for planetary modelling.

Journal Keywords: Ambient Temperature; Ammonia; Ammonium Compounds; Bcc Lattices; Compression; Hydrates; Phase Diagrams; Phase Stability; Pressure Dependence; Temperature Dependence; Water

Subject Areas: Chemistry, Physics

Instruments: I15-Extreme Conditions