The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

454 / 2019-03-09 12:12:08

Exotic Hydrogen Bonding in Compressed Ammonia Hydrides

Hydrogen-rich compounds; CALYPSO; Crystal structures; High pressure

Plenary track > High Pressure Physics and Materials Science

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 using an unbiased swarm structure search method in conjunction with first-principles density functional calculations. We identify two novel ionic phases in an unusual stoichiometry NH7. The first is a hexagonal phase stabilized above 25 GPa. The exotic NH3-H+-NH3 unit comprises two NH3 molecules bound to a proton donated from a H2 molecule; moreover, this structure possesses a peculiar hydrogen bonding between anionic and cationic units. Upon further compression above 60 GPa, the structure transforms to a tetragonal phase comprising NH4+, H- and H2 units. These extraordinary NH7 phases host the highest gravimetric density (19 wt.%) of extractable hydrogen among all known materials. These insights advance fundamental knowledge on the mechanisms underlying the new structure and physics in these remarkable compounds, which may be present inside some large planets and are within the reach of high-pressure synthesis in the laboratory.