High-pressure Synthesis of Cobalt Polynitrides: Unveiling Intriguing Crystal Structures and Nitridation Behavior

Area of Science:

• Materials Science
• High-Pressure Physics
• Solid-State Chemistry

Background:

• The cobalt-nitrogen system is crucial for understanding material behavior under extreme conditions.
• Previous studies have identified some cobalt nitride phases, but high-pressure behavior remains underexplored.

Purpose of the Study:

• To investigate phase stability and synthesize novel cobalt nitrides under high pressures.
• To characterize the structural and bonding properties of newly discovered cobalt polynitrides.

Main Methods:

• High-pressure synthesis using laser-heated diamond anvil cells.
• Characterization via Raman spectroscopy and single-crystal X-ray diffraction.
• Theoretical validation through first-principles calculations.

Main Results:

• Established stability fields for hexagonal NiAs-type CoN and marcasite-type CoN2 (50-90 GPa).
• Synthesized and characterized novel nitrides: Co3N2, Pnma-CoN, CoN3, and CoN5 (90-120 GPa).
• Discovered unique polymeric nitrogen structures in CoN3 (branched trimers) and CoN5 (π-bonded chains).
• Observed a transition in CoN6 polyhedral frameworks from face-sharing to edge-sharing and isolated structures with increasing nitrogen content.

Conclusions:

• The study expands the known phase diagram of cobalt nitrides at high pressures.
• Novel polynitrides with unique nitrogen architectures were discovered, offering new avenues for materials design.
• Insights into structure-property relationships in cobalt nitrides under pressure are provided, relevant for advanced energy materials development.