Exotic Hydrogen Bonding in Compressed Ammonia Hydrides

Hydrogen-rich compounds attract significant fundamental and practical interest for their ability to accommodate diverse hydrogen bonding patterns and their promise as superior energy storage materials. Here, we report on an intriguing discovery of exotic hydrogen bonding in compressed ammonia hydrides and identify two novel ionic phases in an unusual stoichiometry NH7. The first is a hexagonal R (3) over barm phase containing NH3-H+-NH3, H-, and H-2 structural units stabilized above 25 GPa. The exotic NH3-H+-NH3 unit comprises two NH3 molecules bound to a proton donated from a H-2 molecule. Above 60 GPa, the structure transforms to a tetragonal P4(1)2(1) 2 phase comprising NH4+, H-, and H-2 units. At elevated temperatures, fascinating supertonic phases of NH7 with part-solid and part-liquid structural forms are identified. The present findings advance fundamental knowledge about ammonia hydrides at high pressure with broad implications for studying planetary interiors and superior hydrogen storage materials.