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 CoN₂ within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co₃N₂, Pnma-CoN and two polynitrides, CoN₃ and CoN₅, within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN₃ feature branched-type nitrogen trimers (N₃) and CoN₅ show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN₆ polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN₂ and CoN₃), and finally to isolated (in CoN₅). 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.