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Abstract 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‐Fe7N3and γ′‐Fe4N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe7N3and γ′‐Fe4N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe7N3by 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 yieldsV0 = 83.29 ± 0.03 (Å3),K0 = 232 ± 9 GPa,K0′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe7N3above the spin transition completion pressure, andV0 = 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 Fe7C3induces 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.
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Phase Stability and Thermal Equation of State of Iron Carbide Fe 3 C to 245 GPa
Abstract We conducted shock wave experiments on iron carbide Fe3C up to a Hugoniot pressure of 245 GPa. The correlation between the particle velocity (up) and shock wave velocity (us) can be fitted into a linear relationship,us= 4.627(±0.073) + 1.614(±0.028)up. The density‐pressure relationship is consistent with a single‐phase compression without decomposition. The inference is further supported by the comparison of the observed Hugoniot density with the calculated Hugoniot curves of possible decomposition products. The new Hugoniot data combined with the reported 300‐K isothermal compression data yielded a Grüneisen parameter ofγ= 2.23(7.982/ρ)0.29. The thermal equation of state of Fe3C is further used to calculate the density profile of Fe3C along the Earth's adiabatic geotherm. The density of Fe3C was found to be too low (by ~5%) to match the observed density in the Earth's inner core, and Fe3C is unlikely a dominant component of the inner core.
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- Award ID(s):
- 1619868
- PAR ID:
- 10446330
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 20
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 11018-11024
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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