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

In this study, we conduct extensive high-pressure experiments to investigate phase stability in the cobalt-nitrogen system. Through a combination of synthesis in a laser-heated diamond anvil cell, first-principles calculations, Raman spectroscopy, and single-crystal X-ray diffraction, we establish the stability fields of known high-pressure phases, hexagonal NiAs-type CoN, and marcasite-type CoN ₂ within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co ₃ N ₂ , Pnma-CoN and two polynitrides, CoN ₃ and CoN ₅ , within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN ₃ feature branched-type nitrogen trimers (N ₃ ) and CoN ₅ show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN ₆ polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN ₂ and CoN ₃ ), and finally to isolated (in CoN ₅ ). Our study provides insights into the intricate interplay between structure evolution, bonding arrangements, and high-pressure synthesis in polynitrides, expanding the knowledge for the development of advanced energy materials.
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