Phase transition of ammonia quarterhydrate at high pressure

Understanding the phase behavior of ammonia-water hydrates at high pressures is crucial for modeling the interiors of icy planets. In this study, the behavior of ammonia quarterhydrate [$4\mathrm{N}{\mathrm{H}}_{3}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$ (AQH)] has been measured using Raman spectroscopy and x-ray diffraction up to 33.8 GPa at room temperature. Our results reveal that AQH experienced four phase transitions. At 2.8 GPa, liquid AQH transforms into AQH-II, where ammonia and water molecules randomly occupy the lattice sites of the $P{2}_{1}/c$ space group. Within the 2.8--26.4-GPa range, the two twists in the axial ratio indicate that AQH undergoes two types of isostructural transitions. The first, at 14.6 GPa, AQH-II\ensuremath{'} is caused by the rearrangement of hydrogen atoms on ammonia molecules. In the second, at 20.4 GPa, an ionic-molecular phase (AQH-II\ensuremath{'} ionic) results from the ionization of ammonia molecules. Above 26.4 GPa, the AQH-II\ensuremath{'} ionic phase transforms into the cubic disordered ionic-molecular alloy phase.