Polymorphism of polymeric nitrogen at high pressures.

Area of Science:

• Materials Science
• High-Pressure Physics
• Chemistry

Background:

• Polymeric nitrogen exists in cubic gauche (cg-N), layered polymeric (LP-N), and amorphous (η-N) phases at high pressures (120-180 GPa).
• These high-pressure nitrogen phases are of interest for potential high energy density material applications.

Purpose of the Study:

• Investigate the stability of different polymeric nitrogen modifications under high pressure.
• Characterize phase transitions and metastability of nitrogen phases upon decompression from 161 GPa.

Main Methods:

• Utilized a laser-heated diamond anvil cell to synthesize and study nitrogen phases.
• Performed experiments involving compression and decompression up to 161 GPa.
• Observed phase transformations and stability at high temperatures (up to 2500 K) and varying pressures.

Main Results:

• Synthesized pure LP-N above 152 GPa by laser heating η-N.
• Observed cg-N formation below 150 GPa.
• Found that LP-N diminished at lower pressures (129 GPa, 123 GPa), favoring cg-N synthesis in mixed-phase regions.
• Demonstrated metastability of cg-N and LP-N down to 71 GPa at elevated temperatures and room temperature, respectively.

Conclusions:

• Clarified the complex polymorphism of monatomic nitrogen at high pressures.
• Highlighted significant hysteresis phenomena associated with the transition to nonmolecular nitrogen.
• Provided insights into the conditions for synthesizing and stabilizing different nitrogen phases.