The structure of liquid alkali nitrates and nitrites

Abstract

High energy X-ray diffraction has been combined with containerless techniques to determine the structure of a series of alkali and ammonium nitrate and nitrite liquids. The systems have been modelled using molecular dynamics simulation which allows for the flexibility of, and movement of charge within, the molecular anions. The model reproduces the experimentally-determined scattering functions in both the low- and high-Q regimes reflecting the inter- and intra-molecular length-scales. For ammonium nitrate the best fit to the diffraction data is obtained by assuming the NH₄⁺ cation to have a radius closer to that for Cs⁺ rather than a smaller cation such as Rb⁺ as often previously assumed. The alkali nitrites show an emergent length scale, attributed to the nitrogen–nitrogen spatial correlations, that depends on both temperature and the identity of the alkali cation. The corresponding nitrates show a more subtle effect in the nitrogen–nitrogen correlations. As a result, the nature of this N–N length-scale appears different for the respective nitrites and nitrates.