Fig. 1. Ideal crystal structures of (a) γ-Fe₄N, (b) γ-Fe₃NiN, (c) γ-FeNi₃, and (d) ε-Fe₃N. The blue and green balls represent Fe and Ni atoms, respectively, and the red ball the N atom.

Fig. 2. (a) Typical reaction product from an HPSSM experiments. (b) In the reactions, nitrogen degassing leads to abundant cavities. (c) and (d) SEM images of typical iron-nickel nitride samples.

Fig. 3. (a) HAADF-STEM image of an iron–nickel nitride. (b) The corresponding SAED pattern, where the spots corresponding to phases A, B, and C are represented in red, blue, and green, respectively. (c) Schematic diagram of the SAED pattern, with each color corresponding to a crystal structure. (d)–(f) Corresponding STEM-EDX elemental maps showing the distributions of Fe, Ni, and N, respectively, with A, B, and C representing γ-FeNi₃-type, γ-Fe₄N-type, and ε-Fe₃N-type nitrides, respectively.

Fig. 4. Rietveld refinement (red line) applied to micro-focused XRD patterns acquired from iron–nickel nitride samples.

Fig. 5. Plots of normalized unit-cell volume vs pressure for iron–nickel nitrides. The inset shows Rietveld refinement patterns from synchrotron radiation ADXRD of iron–nickel-based nitride powder under a pressure of 1.87 GPa.

Fig. 6. (a) Rietveld-refined NPD patterns for iron–nickel nitrides. (b), (c), and (d) Crystal structures of three iron–nickel nitrides as determined from NPD refinement, corresponding to phases A, B, and C, respectively. For phases A and B, the Fe and Ni atoms simultaneously occupy the cation sites (3c and 1a), and the N atom occupies the 1b site. For phase C, the Ni atoms partially substitute for Fe atoms at 6g sites, and the N atoms diffuse from 2c (red) to 2b (yellow) and 2d (brown) sites.

Table 1. Elemental compositions of the three phases as determined by EDS, NPD, and XRD.

Table 2. Space group, lattice parameters a and c, and unit-cell volume V of the three phases for iron–nickel nitrides and reference phases.

Table 3. Experimental and calculated bulk modulus B₀ and its first derivative B0′ at zero pressure for iron–nickel nitrides.

Table 4. Structural information on the three phases within iron–nickel nitrides obtained from the Rietveld-refined NPD patterns, with R_wp = 30.2% and χ² = 2.09.