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Iron ii nitride (FeN) is an important and well-studied compound of the transition metal pernitrides, exhibiting superconductivity and magnetism at low temperature. However, it is still not clear how nitrogen (N) species affect the physics of this material under high pressure conditions.
Previous structural studies have revealed that at low pressures, FeN crystallizes in a trigonal R-3m structure11, while above this threshold, an orthorhombic Pnnm phase becomes more favorable21,22. Both structures show N structural features, such as a dinitrogen unit and N-sharing six-fold FeN6 octahedrons (Figs S11,12).
However, these N features cannot explain the strong magnetism of FeN under HP. To address this issue, we performed extensive structure searches for stoichiometric Fe-N compounds using the CALYPSO unbiased particle swarm optimization algorithms for high-pressure prediction18.
We have found that in the N-rich Cmmm phase at 250 GPa, each N atom forms an s bond with two adjacent Fe atoms and three neighboring N atoms, and a 6x6x6 coordination decahedron with Np octahedra. This structure has N symmetry of 2c. The ELF plots of the Cmmm phase display a striking pattern: whereas 6x6x6 coordination characterizes the N-sharing octahedrons in the P-1 structure, 8x8x8 coordinate decahedra appear in the Cmmm phase.
This discovery of an N-rich structure with N symmetry indicates that nitrogen in the system is not just a simple substitution site for hydrogen. It also provides a new avenue to study the physics of N-rich nitrides under high-pressure conditions. This could open up a whole new class of materials with unusual properties.