A polymeric nitrogen N[Formula: see text]-N[Formula: see text] system with enhanced stability at low pressure

Area of Science:

• Materials Science
• Chemistry
• Physics

Background:

• The singly-bonded nitrogen cubic gauche crystal (cg-PN) is a leading high energy density material (HEDM) but requires extreme synthesis conditions (2000 K, 110 GPa).
• Continued research seeks HEDMs synthesizable under milder conditions.

Purpose of the Study:

• To identify novel polynitrogen structures with high energy density and improved synthesizability.
• To evaluate the stability and energetic properties of predicted nitrogen-based molecular crystals.

Main Methods:

• Utilized ab initio evolutionary crystal prediction techniques.
• Assessed dynamical and mechanical stabilities at 0 and 5 GPa.
• Calculated vibrational frequencies for Raman and IR spectra.

Main Results:

• A simpler nitrogen-based molecular crystal composed of N4 and N2 molecules was identified as a favorable polynitrogen at lower pressures.
• This N4-N2 system exhibits an energetic gain of 534 meV/atom over cg-PN and 138 meV/atom over the N4 molecular crystal at zero pressure.
• The predicted crystal structure demonstrates dynamical and mechanical stability.

Conclusions:

• The N4-N2 molecular crystal presents a more appealing high energy density material candidate due to its lower synthesis pressure requirements and higher energetic gain.
• Its predicted structure, featuring N4 molecules with D4h symmetry, suggests a higher likelihood of experimental synthesis compared to cg-PN.