Pressure-induced ionization and hydrogen-bond symmetrization of ammonia hydrates: implications for the magnetic-field architectures of ice giants

Water and ammonia are of vital importance in planetary science and are regarded as the main constituents of icy giants (Uranus and Neptune) as well as of icy satellites such as Titan, Triton, and the dwarf planet Pluto. In addition, high-pressure ionic phase-transition studies of ammonia and water are particularly crucial for verifying the physical feasibility of the magnetic-field models of icy giants—models in which the field is dominated by a quadrupole term rather than the dipole term seen in other Solar-System planets. Some previous studies have shown that both water ice and ammonia ice undergo ionic phase transitions under high pressure, whereas investigations of the ionic phase-transition behaviour of ammonia–water mixtures at pressures beyond the commonly encountered DMA phase are scarce. In this study, high-pressure Raman scattering and X-ray diffraction are employed to investigate the ionizing phase transitions of ammonia hydrates of different concentration ratios up to 202 GPa, and the transition mechanisms together with their variation with concentration are summarized. The experiments extend high-pressure investigations of ammonia hydrates of different concentrations into a new pressure range, elucidate two phase-transition mechanisms—ionization and hydrogen-bond symmetrization—occurring in ammonia hydrates under high pressure, provide fresh experimental evidence for pressure-driven proton motion, and offer new insights into the study of ionic and superionic phases of the ammonia–water and related mixture systems.