Spin transitions and compressibility of ε-Fe7N3 and γ’-Fe4N: implications for iron alloys in terrestrial planetary cores

Abstract:

Iron nitrides are possible constituents in the cores of the Earth and other terrestrial planets. Pressure-induced magnetic changes in iron nitrides and their effects on elasticity remain poorly understood. Here we report synchrotron X-ray emission spectroscopy (XES) and X-ray diffraction (XRD) data of ε-Fe₇N₃ and γ’-Fe₄N compressed up to 60 GPa at 300 K using diamond anvil cells. The XES spectra reveal completion of high- to low-spin state transition in ε-Fe₇N₃ and γ’-Fe₄N within the experimental pressure range. The spin transition induces elastic stiffening in ε-Fe₇N₃ but has no resolvable effect on the compression behavior of γ’-Fe₄N. We re-examine 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, illustrating composition dependence of the pressure and effects of high- to low-spin transition. The changes in incompressibility induced by spin transition are important for evaluating the role of light element in iron-rich cores.