Transformation pathways in high-pressure solid nitrogen: from molecular N2 to polymeric cg-N

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Chemistry

Background:

• Understanding high-pressure phases of nitrogen is crucial for materials science.
• Previous studies have explored various nitrogen allotropes under extreme conditions.

Purpose of the Study:

• To investigate the transformation pathway of solid nitrogen from molecular N2 to polymeric cg-N.
• To elucidate the intermediate states and mechanisms governing this high-pressure phase transition.

Main Methods:

• Ab initio molecular dynamics simulations.
• Metadynamics simulations at high pressures (110-120 GPa) and temperature (1500 K).

Main Results:

• Observed a transformation mechanism initiated by trans-cis chain formation from the Immm phase.
• Identified a mixed chain-planar state with Pnma symmetry as an intermediate.
• Discovered two new nitrogen phases: molecular P2(1)/c and chain-like Cm.
• Characterized the pathway as progressive polymerization through variously connected trans-cis chains.

Conclusions:

• Trans-cis chains are the fundamental structural units driving the molecular-polymeric transformation in compressed nitrogen.
• The final cg-N phase forms through collective rearrangement and bond formation in layered chains.
• This study provides detailed insights into the complex phase transitions of nitrogen under extreme pressure.