Stabilization of N and N anionic units and 2D polynitrogen layers in high-pressure scandium polynitrides.

Nitrogen catenation under high pressure leads to the formation of polynitrogen compounds with potentially unique properties. The exploration of the entire spectrum of poly- and oligo-nitrogen moieties is still in its earliest stages. Here, we report on four novel scandium nitrides, ScN, ScN, ScN and ScN, synthesized by direct reaction between yttrium and nitrogen at 78-125 GPa and 2500 K in laser-heated diamond anvil cells. High-pressure synchrotron single-crystal X-ray diffraction reveals that in the crystal structures of the nitrogen-rich ScN, ScN and ScN phases nitrogen is catenated forming previously unknown N and N units and anionic corrugated 2D-polynitrogen layers consisting of fused N rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that ScN and ScN possess an anion-driven metallicity, while ScN is an indirect semiconductor. ScN, ScN, and ScN solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.