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 R3̅ m phase containing NH₃-H⁺-NH₃, H^-, and H₂ structural units stabilized above 25 GPa. The exotic NH₃-H⁺-NH₃ unit comprises two NH₃ molecules bound to a proton donated from a H₂ molecule. Above 60 GPa, the structure transforms to a tetragonal P4₁2₁2 phase comprising NH₄⁺, H^-, and H₂ units. At elevated temperatures, fascinating superionic 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.