Unveiling the Phase Evolution of Quaternary Iron-Group Metal Nitrides through High-Pressure Directed Synthesis

The synthesis of bulk iron-group metal nitrides has long been constrained by thermodynamic challenges and difficulties in controlling crystal structure. In this study, we present a high-pressure directed synthesis (HDS) strategy, using cobalt as a structure-directing agent, to overcome these barriers. Leveraging cobalt's inherent fcc stability, HDS regulates phase formation pathways and energy barriers for directed crystal structure control. Using this approach, we successfully synthesized a single-phase, millimeter-scale bulk quaternary nitride γ-(Fe_0.161Co_0.713Ni_0.126)N_0.071 with an fcc structure for the first time under 5 GPa and 1573 K. Neutron powder diffraction confirms the Fm3̅m structure, with Fe, Co, and Ni randomly occupying the 4a sites and N atoms at the 4b Wyckoff sites. In situ high-pressure synchrotron angle-dispersive X-ray diffraction (ADXRD) measurements determined the sample's bulk modulus (B₀) to be 162 GPa. Moreover, primarily due to interstitial nitrogen solid-solution strengthening, the material exhibits a high Vickers hardness of 373.6 HV₁₀, significantly surpassing that of common austenitic steels. In addition, it exhibits excellent soft magnetic properties with a saturation magnetization of 137 emu·g^-1 and a low coercivity of 8.23 Oe. Overall, this work proposes a new paradigm for the structural design and synthesis of multicomponent nitrides.