Structure of liquid ammonia at high pressures and temperatures

Abstract

The structure of liquid ammonia (NH3) is investigated from 1 to 6.3 GPa and up to 800 K by means of synchrotron x-ray diffraction (XRD) and ab initio molecular dynamics (AIMD) simulations. The XRD data are used to extract the molecular structure factor S mol (Q), pair distribution function (PDF) g mol (r), and the density of NH3. There is an excellent agreement between present S mol (Q) and g mol (r) at our lowest density and those reported in reference neutron experiments. Our densities agree better with the equation of state of Tillner-Roth et al. [1] than with more recent equation of state (EoS) models. The experimental structure factor and PDF are well reproduced by AIMD simulations using either the BLYP or the PBE exchange-correlation functional. The shapes of S mol (Q) and g mol (r) vary little over the investigated pressure range and suggest a compact liquid with weak orientational correlations between molecules, which is corroborated by the coordination number varying from 12.7 to ∼14. The simulations are used to study the evolution of the site-site pair distribution functions, which reveals that the number of H-bonds per molecule (between 1.5 and 2) do not evolve with density, and that the distribution of H atoms around N atoms becomes more and more anisotropic with pressure.

Electrofreezing of Liquid Ammonia

“…In particular, not only the spatial extent of the first solvation shell of the ammonia molecules is expanded, but also the population of first-neighboring species increases upon the field action, as reported in the inset of Figure 3 . In fact, starting from a coordination number of 12.0 in the zero-field case ( vis-à-vis an experimental value of 12.7 49 ), it reaches a maximum value of ∼15.7 for a field strength of 0.40 V/Å, which decreases to ∼5.9 above this field threshold. On the other hand, it has to be noticed that the first local minimum of the NN RDF at strong field regimes exhibits a height larger than 1 and that the integral up to the second (far deeper) minimum leads to a coordination number equal to ∼11.1, as displayed in Figure S8 of the SI.…”

Quantum cluster equilibrium theory applied to liquid ammonia

“…27 Furthermore, Calliol et al found that the radial distribution function curves by the hypernetted chain (HNC) and the reference hypernetted chain are found to be in good accordance with experimental data 25 for the nitrogen-nitrogen distribution function at 4 ∘ C. 28 Besides, Queyroux et al, in their paper investigating at high pressures and temperatures the structure of liquid ammonia by synchrotron X-ray diffraction, reported that above 1 GPa and up to 6.3 GPa at 800 K ammonia structures revealed a close packing form. 29 Characterization of ammonia structure in the liquid phase by radial distribution function done by Gao et al exhibits a formation of an average of three hydrogen bonds per ammonia molecule, and the liquid is made up of winding chains of hydrogen-bonded monomers. Roughly linear hydrogen bonds predominate in the liquid phase.…”

Metallization of Shock-Compressed Liquid Ammonia

“…Despite its importance, experimental studies of the high pressure behavior of ammonia remain very scarce and limited to a narrow thermodynamic range. Measurements in diamond anvil cells have characterized the equation of state and phase diagram up to 200 GPa at 300 K, but reached at most ∼40 GPa at higher temperatures (∼3000 K) [29][30][31][32][33][34][35][36]. Few studies have been carried out employing dynamic compression methods, mainly at gasgun facilities, providing shock compression data up to 70 GPa [37][38][39][40][41][42].…”