Thermoelasticity of Fe₇C₃ under inner core conditions

Abstract

It has recently been reported, on the basis of extrapolated experimental data, that the iron carbide, Fe7C3, has shear wave velocities and a Poisson's ratio consistent with the seismological values for the Earth's inner core and thus that Fe7C3 is a strong candidate for the inner core composition. In this study, using ab initio molecular dynamics simulations, we report the thermoelastic properties of Fe7C3 at 350 GPa up to its melting temperature. Due to significant elastic softening prior to melting, the calculated elastic properties, including wave velocities, do indeed agree well with those from seismology. However, the density was found to be much too low (by ~8%) when compared to geophysical data, and therefore, Fe7C3 must be ruled out as a major component of the Earth's inner core.

First‐principles prediction of Si‐doped Fe carbide as one of the possible constituents of Earth's inner core

“…Furthermore, recent experimental results also suggest that o‐Fe 7 C 3 [ Prescher et al ., ] is a potentially important phase for interpretation of the properties of the Earth's core. However, results obtained from ab initio molecular dynamics simulations claim that the density of Fe 7 C 3 can have a too low density relative to the inner core [ Li et al ., ]. Recent inelastic X‐ray scattering measurements of liquid iron‐carbon alloy show that V P is substantially slower than a phase consisting of pure Fe and Fe 3 C and faster than that of liquid Fe [ Nakajima et al ., ].…”

High pressure thermoelasticity and sound velocities of Fe-Ni-Si alloys

“…However, cosmochemical and iron-silicate melt partitioning studies have suggested the carbon content of the whole core is <0.7-1 wt% (Wood et al, 2013;Zhang and Yin, 2012). An ab initio study suggested the density of Fe 7 C 3 was too low to be a substantial component of the inner core (Li et al, 2016), and, in the analysis of equations of state and seismic observations, Morrison et al (2018) find a carbon content of less than 1 wt%. FeO and iron-rich (Mg,Fe)O also have a low v S compared to that of hcp-Fe, suggesting a large oxygen content could explain the low v S of the inner core.…”

New high-pressure phases of Fe7N3 and Fe7C3 stable at Earth's core conditions: evidences for carbon–nitrogen isomorphism in Fe-compounds

“…The close correspondence of the h-phase elastic properties and those of PREM (preliminary reference Earth model) 5 at 350 GPa and 6500 K was shown with molecular dynamic simulations. 19 However, the obtained density is signicantly lower than that of PREM. With evolutionary crystal structure prediction method, which we also applied here, it was shown that the h-Fe 7 C 3 phase is the most energetically favourable modication in the pressure range of 100-400 GPa at 0 K. 20 Fe 7 N 3 was synthesized in the form of the low-pressure 3and high-pressure b-phases.…”