"Invar"-like behavior in compressed Fe7C3 with implication for deep carbon cycle

Abstract:

Iron carbide Fe₇C₃ has recently emerged as a leading candidate component of the inner core because it is likely the first phase to solidify from a liquid containing iron and a small amount of carbon, and previous studies suggest that it provides a good match for the density of the inner core under relevant conditions. Pressure-induced magnetic transitions have been observed in Fe₇C₃ (Chen et al., 2012). The pressure of the ferromagnetic to paramagnetic transition remains controversial and its effect on equation of state (EoS) is unclear, thus introducing uncertainties in estimating the density of Fe₇C₃ under inner core pressures.

Here we report the lattice parameters and unit cell volume of hexagonal Fe₇C₃ at 300 K and up to 70 GPa, obtained through synchrotron x-ray diffraction measurements using a diamond anvil cell. The experiments used fine powder of Fe₇C₃ that was synthesized in the multi-anvil apparatus at the University of Michigan. The sample was embedded in neon pressure medium together with Au powder and ruby spheres as additional pressure markers. We observed significant softening at 5~8 GPa, similar to the reported "invar"-like behavior in Fe-Ni alloy (Dubrovinsky et al., 2001). For comparison, the compression curve of iron in the same loading turned out to be smooth as expected, which confirms that the abnormal behavior in Fe₇C₃ compression curve is due to its own property change and not an artifact.

The new data allow us to establish the equation-of-state (EoS) of Fe₇C₃ and then estimate the density of Fe₇C₃ at inner core conditions.

References:

Chen, B., Gao, L.L., Lavina, B., Dera, P., Alp, E.E., Zhao, J.Y., Li, J., 2012. Magneto-elastic coupling in compressed Fe₇C₃ supports carbon in Earth's inner core. Geophys Res Lett 39.

Dubrovinsky, L., Dubrovinskaia, N., Abrikosov, I.A., Vennstrom, M., Westman, F., Carlson, S., van Schilfgaarde, M., Johansson, B., 2001. Pressure-induced invar effect in Fe-Ni alloys. Phys Rev Lett 86, 4851-4854.