Experimental Pathways for Detecting Double Superionicity in Planetary Ices

The mantles of Uranus, Neptune and sub-Neptune exoplanets are likely rich in planetary ices water, methane, and ammonia. Under extreme pressures and temperatures in the planetary interiors, these materials form complex mixtures and assume a superionic state, in which the hydrogen atoms diffuse like a liquid through a stable lattice of the heavier nuclei. Recent computational work [1] has predicted the existence of second type of superionic state, in which hydrogen and one of the heavier atoms diffuse through the crystal simultaneously. This doubly superionic state was predicted for many H-C-N-O materials. Here we propose an experimental pathway for reaching and detecting such a doubly superionic state. Starting from a liquid mixture of H₂O and HNO₃, we propose shock compression experiments to reach condition of high pressure and temperature where H₃NO₄ is doubly superionic. Then we suggest employing X-ray diffraction measurements to detect the vanishing of nitrogen peaks upon the transition to the doubly superionic phase, while the peaks that are associated with the oxygen sublattice remain in place.

[1] K. de Villa, F. González-Cataldo, B. Militzer, "Double superionicity in icy compounds at planetary interior conditions", Nature Communications 14 (2023) 7580.