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Spin Transitions and Compressibility of ε‐Fe 7 N 3 and γ′‐Fe 4 N: Implications for Iron Alloys in Terrestrial Planet Cores
Author(s) -
Lv Mingda,
Liu Jiachao,
Zhu Feng,
Li Jie,
Zhang Dongzhou,
Xiao Yuming,
Dorfman Susannah M.
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2020jb020660
Subject(s) - bulk modulus , compressibility , nitride , spin transition , materials science , analytical chemistry (journal) , spin states , spin (aerodynamics) , crystallography , chemistry , condensed matter physics , thermodynamics , physics , nanotechnology , layer (electronics) , chromatography
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‐Fe 7 N 3 and γ′‐Fe 4 N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe 7 N 3 and γ′‐Fe 4 N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe 7 N 3 by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe 4 N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yields V 0 = 83.29 ± 0.03 (Å 3 ), K 0 = 232 ± 9 GPa, K 0 ′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe 7 N 3 above the spin transition completion pressure, and V 0 = 54.82 ± 0.02 (Å 3 ), K 0 = 152 ± 2 GPa, K 0 ′ = 4.0 ± 0.1 for γ′‐Fe 4 N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe 3 S, Fe 3 P, Fe 7 C 3 , and Fe 3 C 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 Fe 3 S, Fe 3 P, and Fe 3 C induces elastic stiffening, whereas that of Fe 7 C 3 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.