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.

本项目旨在通过10-20 GPa、300-700 K温压区间内的单晶同步辐射X射线衍射（synchrotron XRD）技术，解析氨合物高压固相的晶体结构。氨合物作为形成氢键的典型化合物，在基础科学领域极具研究价值；同时作为冰质行星的主要组成成分，亦是行星科学的关键研究对象。精准限定天体内部结构模型，需获取不同组分的H₂O/NH₃混合物在宽温压范围内的实验数据。此前我们已针对三种氨合物开展了50 GPa高压、800 K高温条件下的相图研究，并发现了一项共性规律：三种氨合物中均存在“准体心立方（quasi-bcc）”相。目前此类相的晶体结构、氧/氮（O/N）与氢（H）的有序排布仍不明确，本项目将通过单晶X射线衍射实验对其展开研究。