Light elements in the core:: Effects of impurities on the phase diagram of iron

The stable phase of iron ( Fe) in the Earth's core has been the subject of extensive studies, yet still remains controversial. Hexagonal close-packed structure (hcp) is the accepted stable form of pure Fe; however, it has been recently proposed that a body-centered-cubic (bcc) structured alloy of Fe with lighter elements would be a strong candidate. Experiments and ab initio calculations have shown that small amounts of silicon can stabilize the bcc phase with respect to the hcp phase. In this study we examine the effect at core conditions by adding small concentrations of carbon ( C), sulphur ( S) and oxygen (O) to iron using electronic structure calculations. We analyze the effect of the light elements on the energy difference between the two structures, and we conclude that the bcc phase tends to be stabilized by the alloys, strengthening the case for a bcc-structured inner core of the Earth.