Geophysical and cosmochemical models constrain the inner core to be composed mainly of iron and few percent light elements including Si, S, C, and H. However, mineral physics results do not agree with the seismological observations of both density and sound wave velocities in the inner core (Vočadlo, 2007; Vočadlo et al., 2009; Belonoshko et al., 2007; Martorell et al., 2013). Recent studies suggest a candidate for the inner core could be hcp iron alloyed with light elements (Li. et al., 2001; Antonangeli, et al., 2010; Hirose, et al., 2013). Using ab initio molecular dynamics calculations, we have studied the structure and elastic properties of hcp-Fe alloys at 360 GPa up to the melting temperature. A ternary hcp-Fe30Si1C1 alloy was found to match both the inner-core density and sound velocities. We also found other possible ternary and quaternary candidates based on a solid solution model, where carbon is always a necessary component to be in agreement with the seismic observations. These Fe-C-X compositions provide a new understanding of the Earth's core with important geodynamic implications.

Vočadlo, L., (2007). Ab initio calculations of the elasticity of iron and iron alloys at inner core conditions: evidence for a partially molten inner core? Earth. Planet. Sci. Lett., 254 (1), 227.

Vočadlo, L., Dobson, D. P., Wood, I. G., (2009). Ab initio calculations of the elasticity of hcp-Fe as a function of temperature at inner-core pressure. Earth. Planet. Sci. Lett., 288 (3), 534.

Belonoshko, A. B., Skorodumova, N. V., Davis, S., Osiptsov, A. N., Rosengren, A., Johansson, B., (2007). Origin of the low rigidity of the Earth's inner core. Science, 316 (5831), 1603.

Martorell, B., Vočadlo, L., Brodholt, J., Wood, I. G., (2013b). Strong premelting effect in the elastic properties of hcp-Fe under inner-core conditions. Science 342 (6157), 466.

Li, J., Fei, Y., Mao, H. K., Hirose, K., Shieh, S. R. (2001). Sulfur in the Earth's inner core. Earth. Planet. Sci. Lett., 193(3), 509.

Antonangeli, D., Siebert, J., Badro, J., Farber, D. L., Fiquet, G., Morard, G., Ryerson, F. J. (2010). Composition of the Earth's inner core from high-pressure sound velocity measurements in Fe–Ni–Si alloys. Earth. Planet. Sci. Lett., 295(1), 292.

Hirose, K., Labrosse, S., Hernlund, J., (2013) Composition and state of the core. Annu. Rev. Earth Planet. Sci., 41, 657.