All-Single Bonds Fused N₁₈ Macro-Rings and N₈ Cagelike Building Blocks Stabilized in Lanthanum Supernitrides

Single-bonded polymeric nitrogen has gained tremendous research interest because of its unique physical properties and great potential applications. Despite much progress in theoretical predictions, it is still challenging to experimentally synthesize polynitrogen compounds with novel all-single-bonded units. Herein, we have synthesized two brand-new lanthanum supernitrides LaN₈, through a direct reaction between La and N₂ in laser-heated diamond anvil cells at megabar pressures. Our experiments and calculations revealed that two LaN₈ phases had the <i>R</i>-3 and <i>P</i>4/<i>n</i> symmetry characterized by a unique 2D network with N₁₈ macro-rings and cagelike N₈ building blocks, respectively. Differing from known polynitrogen structures, these two polymers were composed of single-bonded nitrogen atoms belonging to sp³ and sp² hybridizations. In particular, <i>P</i>4/<i>n</i> LaN₈ possessed the longest N-N bond length among all of the experimentally reported metal nitrides, potentially being a high-energy-density material. The present study opens a fresh, promising avenue for the rational design and discovery of new supernitrides with unique nitrogen structures via the high-pressure treatment.