Si and C are cosmochemically abundant elements soluble in hcp Fe under pressure and temperature and could therefore be present in the Earth's inner core. While recent ab initio calculations suggest that the observed inner core density and velocities could be matched by an Fe‐C‐Si alloy, the combined effect of these two elements has only recently started to be investigated experimentally. We therefore carried out synchrotron X‐ray diffraction measurements of an hcp Fe‐C‐Si alloy with 4 at% C and 3 at% Si, up to ∼150 GPa. Density functional theory calculations were also performed to examine different incorporation mechanisms. These calculations suggest interstitial C to be more stable than substitutional C below ~350 GPa. In our calculations, we also find that the lowest‐energy incorporation mechanism in the investigated pressure range (60–400 GPa) is one where two C atoms occupy one atomic site; however, this is unlikely to be stable at high temperatures. Notably, substitutional C is observed to decrease the volume of the hcp Fe, while interstitial C increases it. This allows us to use experimental and theoretical equations of state to show unambiguously that C in the experimental hcp Fe‐C‐Si alloys is not substitutional, as is often assumed. This is crucial since assuming an incorrect incorporation mechanism in experiments leads to incorrect density determinations of ~4%, undermining attempts to estimate the concentration of C in the inner core. In addition, the agreement between our experiments and calculations supports Si and C as being light elements in the inner core.

中文翻译：

---

hcp Fe-C-Si合金的状态方程及C掺入机制对hcp Fe合金在300 K时密度的影响

Si和C是在压力和温度下可溶于hcp Fe的宇宙化学元素，因此可能存在于地球的内核中。尽管最近的从头计算表明Fe-C-Si合金可以使观察到的内芯密度和速度相匹配，但这两种元素的组合作用只是最近才开始进行实验研究。因此，我们对hcp进行了同步加速器X射线衍射测量Fe-C-Si合金具有4 at％的C和3 at％的Si，最高可达〜150 GPa。还进行了密度泛函理论计算以检查不同的结合机理。这些计算表明，间隙C在〜350 GPa以下比替代C更稳定。在我们的计算中，我们还发现，在所研究的压力范围（60-400 GPa）中，最低能量掺入机制是其中两个C原子占据一个原子位点的机制。但是，这在高温下不太可能稳定。值得注意的是，观察到取代C减少了hcp Fe的体积，而间隙C则增加了它的体积。这使我们能够使用实验和理论状态方程来明确显示实验hcp中的CFe-C-Si合金不是通常所假定的替代品。这是至关重要的，因为假设实验中错误的掺入机制会导致〜4％的错误密度测定，从而破坏了估算内核中C浓度的尝试。另外，我们的实验和计算之间的一致性支持Si和C作为内核中的轻元素。