Ammonia molecule rotation of pressure-induced phase transition in ammonia hemihydrates 2NH3·H2O

Abstract

High-pressure Raman scattering and synchrotron angle-dispersive X-ray diffraction studies have been performed on liquid ammonia hemihydrates (2NH₃·H₂O) at room temperature up to 41.0 GPa. The results demonstrate that liquid 2NH₃·H₂O transforms into a solid phase at 3.5 GPa. Upon increasing pressure, a solid-solid phase transition is observed at about 19.0 GPa. When pressure is increased up to 25.8 GPa, another solid-solid phase transition is obtained. The first solid-solid phase transition at about 19.0 GPa originates from the rotation of type II ammonia molecule via the O–H⋯N bond, and this phase transition is from orthorhombic to body-centered-cubic. High-pressure Raman scattering and X-ray diffraction results of 2NH₃·H₂O provide significant information for better understanding the physical properties of the ammonia–water binary system under extreme conditions, and further for the structure state of the outer planets and large satellites in the solar system.