Structure determination of the high-pressure phases of ammonia hydrates by single-crystal diffraction

The aim of this proposal is to solve the structure of the high-pressure solid phases of ammonia hydrates through single-crystal synchrotron XRD in the 10-20 GPa, 300-700 K range. Ammonia hydrates are fascinating systems for fundamental sciences as archetype compounds forming hydrogen (H) bonds but also for planetary sciences as major components of the icy planets. Constraining the interior structure of celestial bodies requires information on H2O/NH3 mixtures of various compositions and over a large range of pressure (P) and temperature (T). We have investigated the high P [50 GPa]-high T [800K] phase diagrams of the three ammonia hydrates and discovered a general behavior, in particular the occurrence in three hydrates of “quasi-bcc” phases. The structure, O/N and H ordering of these phases are still unknown and will be studied in this proposal by single-crystal XRD experiments.