Pressure-induced dehydration and the structure of ammonia hemihydrate-II

C. W. Wilson, C. L. Bull, G. Stinton, J. S. Loveday

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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) angstrom, b = 9.215(1) angstrom, c = 8.933(1) angstrom and beta = 94.331(8)degrees at 3.5(1) GPa. The atomic arrangement has a crowned hexagonal arrangement and is a layered structure with long N-D center dot center dot center dot 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. (C) 2012 American Institute of Physics. []

Original languageEnglish
Article number094506
Pages (from-to)-
Number of pages8
JournalThe Journal of Chemical Physics
Issue number9
Publication statusPublished - 7 Mar 2012


  • ammonia
  • crystal structure
  • dissociation
  • hydrogen bonds
  • lattice constants
  • neutron diffraction
  • space groups
  • X-ray diffraction


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