On High Pressure Ammonium Fluoride, Its Ice Analogues, Hydrates and Hydrides.

Ammonium fluoride, NH₄F, is a hydrogen bond-ordered analogue of ice and forms structures equivalent to ice phases at low pressures. A mixed NH4F-H2O system forms a proton ordering network akin to a spin ice system with magnetic monopoles, in a H2O-rich system NH4F defects act as proton disordering agents [1]. The H2O ice phase diagram shows numerous phase transitions and bond symmetrisation under pressure. Only ice structures with bipartite networks may have a defect free NH₄F analogue, and bond symmetrisation will not occur. We therefore explore NH₄F up to 300 GPa by structure searching to compare how far the analogy holds, and report new close packed high-pressure phases of NH₄F that become stable above 80 GPa. We explore NH₄F-rich systems, where non-bipartite water networks may have NH₄F analogues with a small number of H2O ‘defects’. This may lead to the formation of a wide range of 'quasi-ammonium fluoride' phases as well as clathrate hydrate analogues and we test this on the NH₄F-H₂O-H₂ ternary phase diagram. Finally, we expand into the binary NH₃-HF system (NH₄F represents 1:1) as an analogue of the H-O phase diagram and discuss new stable higher fluorides of ammonium.

[1] C. G. Salzmann et al. J. Phys. Chem. C 123, 16486 (2019).