Phase Stability of Ammonia Hydrate at High Pressures: Implication for the Composition and Velocity Structure of Icy Planets

Mantle of some icy planets and satellites such as, Titan, Uranus and Neptune, is expected to be composed by water, ammonia and methane. Experimental studies on the physical properties of these components at high pressures are of great importance in understanding the physical states and internal structure of these planets and satellites. In this study, we have investigated the phase stability and sound velocity of ammonia-water mixture with various water contents at high pressures and 300 K using both Raman spectra and Brillouin scattering in diamond anvil cells. Although the phase transition of ammonia-water mixture with ratio of 2:1, 1:1, and 1:2 has been studied at high pressures and low temperatures, it is not clear at high pressures and 300 K. More importantly, sound velocity of ammonia-water mixture is unknown. Our high-pressure Raman results have shown that ammonia monohydrate, dihydrate and trihydrate keep in the liquid phase at pressures lower than 2.2 GPa at 300 K. Between 2.3 and 3.6 GPa, all the ammonia-water mixtures decompose into ammonia hemihydrate (2NH₃·H₂O) and ice VII regardless the initial ammonia to water ratio. The liquid ammonia hemihydrate transforms into the solid state at 4.6 GPa and is stable up to 30 GPa. We further determined the sound velocity of ammonia hemihydrate between 2 GPa and 15.3 GPa. P-wave velocity of ammonia hemihydrate increases with pressure from 3.4 to 4.6 km/s between 2 GPa and 4.4 GPa. Furthermore, P-wave velocity of polycrystalline ammonia hemihydrate increases from 5.9 to 8.3 km/s between 4.7 GPa and 15.3 GPa with a velocity slope of 0.23 (km/s/GPa). Our experimental results are needed to constrain the composition and structure of icy planets and satellites.