Polymerization of nitrogen in lithium azide.

Area of Science:

• Solid-state chemistry
• Materials science
• Computational physics

Background:

• Additional electrons significantly alter bonding in light elements, as seen in alkali metal azides forming N3(-) anions instead of N2 molecules.
• The influence of pressure on nitrogen polymerization in azides remains largely unexplored, despite its potential importance.

Purpose of the Study:

• To investigate the structural evolution of lithium azide (LiN3) under extreme pressures.
• To identify new stable phases of LiN3 and understand the role of electron addition and pressure on nitrogen polymerization.

Main Methods:

• Employed first-principles density functional theory (DFT) calculations.
• Utilized the particle swarm optimization (PSO) algorithm for comprehensive structure searching.
• Systematically studied LiN3 structures across a pressure range up to 600 GPa.

Main Results:

• Discovered a novel, stable insulating phase of LiN3 at pressures above 375 GPa.
• This phase features a unique structure with zig-zag nitrogen polymer chains composed of N5(-) five-member rings linked by shared N-N pairs.
• Observed a transition in nitrogen atomic orbital hybridization from sp to sp(2) and finally to sp(3) with increasing pressure.

Conclusions:

• The study elucidates the complete structural evolution of LiN3 under pressure, driven by changes in nitrogen's electronic structure and hybridization.
• The newly identified polymerized nitrogen structure represents a significant finding in high-pressure nitrogen-containing compounds.
• The transition to sp(3) hybridization in nitrogen atoms is key to forming the stable polymeric structure.