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

We perform a computational study based on first-principles calculations to investigate the relative stability and elastic properties of the doped and undoped Fe carbide compounds at 200-364GPa. We find that upon doping a few weight percent of Si impurities at the carbon sites in Fe7C3 carbide phases, the values of Poisson's ratio and density increase while V-P, and V-S decrease compared to their undoped counterparts. This leads to marked improvement in the agreement of seismic parameters such as P wave and S wave velocity, Poisson's ratio, and density with the Preliminary Reference Earth Model (PREM) data. The agreement with PREM data is found to be better for the orthorhombic phase of iron carbide (o-Fe7C3) compared to hexagonal phase (h-Fe7C3). Our theoretical analysis indicates that Fe carbide containing Si impurities can be a possible constituent of the Earth's inner core. Since the density of undoped Fe7C3 is low compared to that of inner core, as discussed in a recent theoretical study, our proposal of Si-doped Fe7C3 can provide an alternative solution as an important component of the Earth's inner core.