Chemical reaction mechanisms of solid state ammonia and hydrogen under high pressure

Ammonia is the most stable compound of nitrogen and hydrogen at ambient pressure. However, the chemical reaction of nitrogen and hydrogen is more complex and difficult to explore at high pressures. Here, we have performed extensively structural searches of ammonia-hydrogen compounds based on particle swarm optimization algorithms and first principles calculations. The calculated results show that the main reaction products of nitrogen and hydrogen under high pressure can be divided into two categories: high-energy density material (HEDM) and hydrogen storage material (HSM). Three different phases of ${\mathrm{NH}}_{4}$ are potential HEDMs, which are found to be stable or metastable at 40 GPa to 300 GPa, and metastable at ambient pressure with energy density of about $2.15\phantom{\rule{0.28em}{0ex}}\mathrm{kJ}/\mathrm{g}\ensuremath{\sim}3.86\phantom{\rule{0.28em}{0ex}}\mathrm{kJ}/\mathrm{g}$. The $Pm$ phase of ${\mathrm{NH}}_{10}$ is an outstanding HSM with ultrahigh hydrogen storage (41.7 wt%) and release (29.2 wt%) capacities. These findings offer significant insights into the structural arrangements and chemical bonding patterns of ammonia-hydrogen compounds at high pressure, and suggest potential experimental avenues for elucidating how diverse metastable structures with distinct properties might be existed in planetary interiors.