Stabilization of N₆ and N₈ anionic units and 2D polynitrogen layers in high-pressure scandium polynitrides.

Abstract

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, Sc₂N₆, Sc₂N₈, ScN_5, and Sc₄N₃, 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 Sc₂N₆, Sc₂N_8, and ScN₅ phases nitrogen is catenated forming previously unknown N₆⁶^- and N₈⁶^- units and <math xmlns="http://www.w3.org/1998/Math/MathML"> <msub><mrow><mspace></mspace></mrow> <mrow><mi>∞</mi></mrow> </msub> <msup><mrow><mspace></mspace></mrow> <mrow><mn>2</mn></mrow> </msup> <mrow><mo>(</mo> <mrow> <msubsup><mrow><mi>N</mi></mrow> <mrow><mn>5</mn></mrow> <mrow><mn>3</mn> <mo>-</mo></mrow> </msubsup> </mrow> <mo>)</mo></mrow> </math> anionic corrugated 2D-polynitrogen layers consisting of fused N₁₂ rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc₂N₆ and Sc₂N₈ possess an anion-driven metallicity, while ScN₅ is an indirect semiconductor. Sc₂N₆, Sc₂N₈, and ScN₅ solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.