Phase Stability and Thermal Equation of State of Iron Carbide Fe₃C 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 of γ = 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.

P-V-Tmeasurements of Fe3C to 117 GPa and 2100 K: Implications for stability of Fe3C phase at core conditions

“…Our EoS for Fe 3 C is compared with reported volume data (Tateno et al 2010;Hu et al 2019;Liu et al 2020). which validates the applicability of our thermal EoS even at the inner core conditions.…”

“…GPa in a multi-anvil experiment and inferred from a shock compression experiment Hu et al 2019;Liu et al, 2020). In order to discuss the density of the core and the P-T location of reaction (1), a self-consistent thermal EoS for Fe 3 C should be established based on higher P-T experimental data.…”

Thermal Equation of State of Fe3C to 327 GPa and Carbon in the Core

“…Our results indicate that the density of Fe 3 C is compatible with seismological data on the inner core, assuming T =~5000 K at the ICB pressure. The present density values for NM Fe 3 C at 5000 K are different from those estimated by Litasov et al [21] based on FM Fe 3 C. The estimation of the high-temperature density based on FM Fe 3 C could be different from that of the real NM Fe 3 C, although the recent density estimation of Fe 3 C based on shock compression [16] is close to the estimation by Litasov et al [21]. In Figure 3b, we show the density of NM Fe 7 C 3 at 5000 K, as described by Chen et al [34] using thermal parameters for the PM phase [35].…”

“…More recently, Takahashi et al [15] studied the stability of Fe 3 C up to and above 300 GPa and showed that it was stable up to this pressure, whereas it melted incongruently into Fe 7 C 3 and liquid under the pressure and temperature conditions studied. The stability of Fe 3 C is also supported by a recent shock experiment by Hu et al [16]. They reported no evidence for decomposition of Fe 3 C to Fe 7 C 3 and Fe in the pressure range from 80 GPa to 248 GPa along the Hugoniot, which is consistent with the results of Mashino et al [14] and Takahashi et al [15].…”

“…Litasov et al [21] determined the compression behavior of ferromagnetic (FM) Fe 3 C up to about 30 GPa and estimated the density of NM Fe 3 C under inner core conditions by assuming that the compression behavior of NM Fe 3 C is similar to that of FM Fe 3 C, which they determined. Very recently, Hu et al [16] conducted shock experiments of Fe 3 C and estimated the density of Fe 3 C under inner core conditions. However, it is not well resolved whether their data correspond to NM Fe 3 C. Presently, there are no high-temperature static compression experiments on NM Fe 3 C.…”