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

Abstract

AbstractIn 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