Electrofreezing of Liquid Ammonia

Abstract

Here we prove that, in addition to temperature and pressure, another important thermodynamic variable permits the exploration of the phase diagram of ammonia: the electric field. By means of (path integral) ab initio molecular dynamics simulations, we predict that, upon applying intense electric fields on ammonia, the electrofreezing phenomenon occurs, leading the liquid toward a novel ferroelectric solid phase. This study proves that electric fields can generally be exploited as the access key to otherwise-unreachable regions in phase diagrams, unveiling the existence of new condensed-phase structures. Furthermore, the reported findings have manifold practical implications, from the safe storage and transportation of ammonia to the understanding of the solid structures this compound forms in planetary contexts.

Figure 1

(a) Infrared (IR) spectra of ammonia at T = 252 K at zero field (black curve) and for different electric field intensities up to a maximum of 0.70 V/Å. (b) Vibrational Stark effect produced by the application of the field on the bending and stretching modes frequencies.

Figure 2

Self-diffusion coefficient of the ammonia molecules as a function of the electric field intensity in logarithmic and (inset) linear scale.

Figure 3

Nitrogen–nitrogen (NN) radial distribution function (RDF) of ammonia for different field intensities from the zero-field regime (black solid curve) up to 0.70 V/Å (red solid curve). Color code follows that of Figure 1a. The inset shows the NN coordination number (black circles) determined as the integral of the RDF up to the first minimum and the location of this latter dip (red squares, red right-handed ordinate axis) as a function of the field strength.

Figure 4

Top panel: order map of ammonia at different electric field intensities (black-to-red gradient scale). Relatively low values of the translational t and orientational q₆ order parameters identify a fluid-like state of ammonia (a liquid), whereas large values of t and q₆ are the manifestation of a field-induced solid-like ammonia structure. Bottom panels: snapshots of the simulation box in the absence (left) and in the presence (right) of the field producing the stacking of the molecules in layers approximately perpendicular to the field direction, the latter being represented by the yellow arrow. In spite of the ferroelectric nature conferred by the field (i.e., all NH₃ molecules are oriented along the field axis), hydrogen atoms remain disordered.