549 / 2019-03-20 14:44:05

High-Pressure Phase of Ammonia Hydrate: Implication for the Internal Structure of Ice Giants

Ammonia Hydrate,Ice-VII,Single-crystal Elasticity,Anisotropy,Ice giant

全体主题 > High Pressure Physics and Materials Science

Recent astronomy observations have identified the existence of hundreds of ice giants with a mean density of ~1 g/cm3 and up to ten times of Earth’s mass. The mantle of these ice giants, including Neptune and Uranus as well as their large satellites, are expected to be composed by the water-ammonia-methane mixture. Experimental studies on the physical properties of the water-ammonia mixture are thus important for understanding the internal structure of ice giants and satellites.
In this study, we have combined Raman and Brillouin spectroscopy to determine the high-pressure phase and elasticity of ammonia hydrates up to 53 GPa and 300 K. All the ammonia hydrates with a H2O to NH3 ratio greater than 1 dehydrate into 2NH3∙H2O (AHH) and single-crystal ice-VII at 4-4.6 GPa and 300 K. Measured sound velocity of AHH indicates that AHH remains stable up to 39 GPa at 300 K, while the single-crystal elasticity of ice-VII exhibits anomalous variation in C12 and C44 at 10-20 GPa and strong softening in both C11 and C12 at 42-53 GPa at 300 K, corresponding to the dipole ordering and the transition to the pre-transitional state of ice-VII, respectively. Of particular interest is the dramatic increase in the anisotropy of ice-VII with increasing pressure. Modeling the velocity of AHH and ice-VII mixture indicates that the mantle of ice giants may have strong anisotropies in velocity.