Liquid Structure of Iron and Iron–Nitrogen–Carbon Alloys Within the Cores of Small Terrestrial Bodies

Abstract

Nitrogen has been proposed to be stored within planetary cores, but its effects on the structure and density of molten Fe–alloys have not been explored experimentally. Using energy-dispersive X-ray diffraction, we determined the structure of Fe–N(–C) liquids at core conditions (1–7 GPa and 1700–1900°C) within a Paris-Edinburgh press. Variation of N up to 7 wt.% and C up to 1.5 wt.% results in near-linear changes in Fe–Fe atom distances and structure factor with increasing light element content. We did not observe a significant pressure-driven structural transition in Fe–N(–C) liquids. We model the expansion of the Fe–Fe bonds using a modified Birch-Murnaghan equation of state. With this model, we demonstrate that N or C contamination could lead to an overestimation of the Fe–Fe distances of pure Fe. We observe that the incorporation of 1 wt.% N or C into Fe results in a change in Fe–Fe distances that is twice as significant as the effect of 1 GPa. By approximating the change in volume, we infer that N and C incorporated in liquid iron could contribute to the density deficit observed in the cores of terrestrial bodies.