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

• Published in: Journal of geophysical research. Solid earth 2020-11, Vol.125 (11), p.n/a
• Main Authors: Lv, Mingda, Liu, Jiachao, Zhu, Feng, Li, Jie, Zhang, Dongzhou, Xiao, Yuming, Dorfman, Susannah M.
• Format: Article
• Language: English
• Subjects: Alloying elements; Alloys; Analytical methods; Bulk modulus; Cementite; Compressibility; Compressibility effects; Compression; Cores; Emission spectroscopy; Equations of state; Extrasolar planets; Ferrous alloys; Geophysics; GEOSCIENCES; Iron; Iron carbides; Iron constituents; Iron nitride; Planetary cores; Planets; Pressure; Spectroscopy; Spectrum analysis; Spin transition; Spin transitions; Stiffening; Synchrotrons; Terrestrial planets; X-ray emissions
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2020JB020660

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 ε‐Fe7N3 and γ′‐Fe4N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe7N3 and γ′‐Fe4N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe7N3 by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe4N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yields V0 = 83.29 ± 0.03 (Å3), K0 = 232 ± 9 GPa, K0′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe7N3 above the spin transition completion pressure, and V0 = 54.82 ± 0.02 (Å3), K0 = 152 ± 2 GPa, K0′ = 4.0 ± 0.1 for γ′‐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 ~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. Key Points Spin transition in ε‐Fe7N3 and γ′‐Fe4N at 300 K completes at 43 and 34 GPa, respectively The completion of spin transition leads to stiffening in bulk modulus of ε‐Fe7N3, but not in γ′‐Fe4N Evidence for spin transitions in Fe‐light‐element alloys and their effects are reexamined