Ammonium fluoride's analogy to ice: Possibilities and limitations

Ammonium fluoride, NH₄F, is often seen as an analog to ice, with several of its solid phases closely resembling known ice phases. While its ionic and hydrogen-ordered nature puts topological constraints on the ice-like network structures it can form, it is not clear what consequences these constraints have for NH₄F compound formation and evolution. Here, we explore computationally the reach and eventual limits of the ice analogy for ammonium fluoride. By combining data mining of known and hypothetical ice networks with crystal structure prediction and density functional calculations, we explore the high-pressure phase diagram of NH₄F and host-guest compounds of its hydrides. Pure NH₄F departs from ice-like behavior above 80 GPa with the emergence of close-packed ionic structures. The predicted stability of NH₄F hydrides shows that NH₄F can act as a host to small guest species, albeit in a topologically severely constraint configuration space. Finally, we explore the binary NH₃-HF chemical space, where we find candidate structures for several unsolved polyfluoride phases; among them is the chemical analog to H₂O₂ dihydrate.