The ices of water, ammonia and methane, present inside Neptune and Uranus, have very different properties to ice existing on Earth: they are hot and have a higher density than their melt.

Ice within the giant planets is submitted to extreme pressures and temperatures, up to 300 GPa, i.e. 3 million atmospheres, and 5000 K. To better understand the properties of these dense "hot" ices, scientists aim to reproduce similar conditions in the laboratory. This is how a new state of ammonia ice, called superionic ammonia, was discovered at high pressure and temperature. Superionicity is an exotic state of matter where the standard concepts of solid and liquid are challenged as the system behaves simultaneously as a crystal (the fixed ion lattice) and as a liquid (the diffusive ions). Superionic solids have intrigued scientists for many years and offer promising applications as components of solid-state batteries, which are currently under investigation. If superionic ice exists inside the giant icy planets Neptune and Uranus, it could be at the origin of the planets magnetic field, which remains unexplained.

At moderate pressures and temperatures, ammonia ice is a molecular crystal where strong covalent bonds coexist with weaker hydrogen bonds, as in water ice. By compressing this solid above ~60 GPa and by annealing above ~750 K, a new form of ammonia ice has been discovered, called the α phase (Figure 1). To study its structure, X-ray diffraction patterns of this new phase were collected at beamline ID09HP. It was deduced from the diffraction patterns that the nitrogen sub-lattice is the same as in the low temperature solid. However, the large entropy variation at the phase transition indicated a large amount of disorder for the hydrogen atoms in the new α phase. To locate the position of the very light H-atoms, hardly visible by X-ray diffraction, the scientists also performed ab initio computer simulations of ammonia samples under the same thermodynamic conditions as in the experiments. The latter revealed that in the α phase the protons are highly mobile and diffuse through the fixed nitrogen lattice (Figure 2). The strong similarities between the experimental observations and the results of calculations showed that the α phase coincides with superionic ammonia ice.

Figure 1. Phase diagram of ammonia. Colour symbols are experimental data (from Raman scattering or X-ray diffraction) and the white symbols are the result of calculations. The structure of phase V is depicted: the green and white spheres represent respectively nitrogen and hydrogen atoms. The nitrogen lattice in phase V is very close to an HCP structure (ABA stacking). H-bonds are shown as dashed lines, distinguishing those linking molecules in the same plane (blue) from those linking molecules across the planes (red).

Such a superionic phase of ammonia had been predicted in a milestone paper [1], but never observed experimentally. The present results thus validate this prediction but locate superionic ammonia at lower temperature than in the theoretical study (750 K instead of 1200 K). The exact boundaries of this superionic phase are of crucial importance as this determines whether such a state could exist in the giant icy planets. To this end, the study will be continued to determine the stability field of superionic ammonia at higher temperatures and pressures.

Figure 2. Illustration of superionic ammonia simulated with ab initio calculations at 70 GPa and 850 K. The set of green atoms shows the fixed, nearly hexagonal lattice. The white spheres represent the 100 different positions of one H atom, time-spaced by 40 fs.

Principal publication and authors

Proton disorder and superionicity in hot dense ammonia ice, S. Ninet, F. Datchi and A.M. Saitta, Phys. Rev. Lett. 108, 165702 (2012).

Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie - Paris 6, CNRS UMR 7590, Paris (France).

References

[1] C. Cavazzoni, G.L. Chiarotti, S. Scandolo, E. Tosatti, M. Bernasconi and M. Parrinello, Science 283, 44 (1999).