Most ammonia is used for fertilizers, but it has also been proposed as a potential carbon-free fuel source. Characterizing structure, in particular in the liquid state, reveals the role of intermolecular bonds in dictating macroscopic properties. EFree Chief Scientist Malcolm Guthrie and other EFRC-funded researchers at the SNAP diffractometer at the Spallation Neutron Source, Oak Ridge National Laboratory have measured diffraction on deuterated ammonia at room temperature. Using the unique capabilities of this instrument, the local structure was measured continuously from the liquid to solid states.
New structural information was interpreted by comparison the known structure of crystalline phase IV. This showed a close match with the local structure of phase III, which had not previously been observed. Moreover, the high-density liquid was highly ordered and showed strong similarities with these crystalline phases.
A peak corresponding to the first H-bond contact was identified in the solid phase III that persisted in the liquid across the pressure range studied.
Despite this continuity of local bonding, a stark increase in the spatial extent of structural correlations was seen in the high pressure liquid. This suggests a fundamental change in the role of the H-bond as density increases. Similar behavior is also observed in a wide range of binary glasses and, surprisingly, in (strongly H-bonded) liquid water, suggesting that the phenomena may be general in nature [M. Guthrie, et al., Phys. Rev. B 85, 184205 (2012)].
Figure: Highly-disordered liquid at low pressure (top) increases its density by becoming increasingly ordered, exhibiting structural correlations out to ~30 Å (bottom, note pseudo layers in liquid). H-bonds are shown in magenta.