Spin Transitions and Compressibility of ε-Fe7N3 and γ′-Fe4N: Implications for Iron Alloys in Terrestrial Planet Cores

Iron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure-induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X-ray emission spectroscopy (XES) and X-ray diffraction (XRD) results for epsilon-Fe7N3 and gamma '-Fe4N up to 60 GPa at 300 K. The XES spectra reveal completion of high- to low-spin transition in epsilon-Fe7N3 and gamma '-Fe4N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of epsilon-Fe7N3 by 22% at similar to 40 GPa, but has no resolvable effect on the compression behavior of gamma '-Fe4N. Fitting pressure-volume data to the Birch-Murnaghan equation of state yields V-0 = 83.29 +/- 0.03 (angstrom(3)), K-0 = 232 +/- 9 GPa, K-0 ' = 4.1 +/- 0.5 for nonmagnetic epsilon-Fe7N3 above the spin transition completion pressure, and V-0 = 54.82 +/- 0.02 (angstrom(3)), K-0 = 152 +/- 2 GPa, K-0 ' = 4.0 +/- 0.1 for gamma '-Fe4N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe3S, Fe3P, Fe7C3, and Fe3C based on previous XES and XRD measurements, we located the completion of high- to low-spin transition at similar to 67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe3S, Fe3P, and Fe3C induces elastic stiffening, whereas that of Fe7C3 induces elastic softening. Changes in compressibility at completion of spin transitions in iron-light element alloys may influence the properties of Earth's and planetary cores.