We determine mass transport and structural properties of binary liquid iron alloys over a wide density (5.055-11.735 gcm(-3)) and temperature range (2,500-6,500 K) using first-principles molecular dynamics. Compositions consist of 96 at% Fe and 4 at% or C, N, and O is negligible for densities below similar to 8 gcm(-3), accompanied by an increase in average coordination numbers to similar to 6, and an increase in mean atomic radii. For densities above similar to 8 gcm(-3), diffusivities and atomic radii of these elements decrease monotonically with pressure, which is typical for the iron-like alloying elements as well as for H, Mg, and S over the whole compression range. While atomic radius ratios move toward unity with compression, diffusivity ratios for the alloying element relative to iron tend to increase for the non-iron-like elements with density. Plain Language Summary The Earth's core is mostly iron with substantial amounts of other elements that dissolved into core-forming metallic melt during the planet's accretion. Although the exact composition of the Earth's core remains unknown, we can obtain important insight into its chemistry and ongoing physical processes by studying the behavior of alloying elements dissolved in liquid iron, such as their transport rates and incorporation mechanisms. Here we investigate the behavior of eight elements (hydrogen, carbon, nitrogen, oxygen, sulfur, silicon, magnesium, and nickel) that are relevant candidates for the Earth's core composition using molecular dynamic simulations, which allow extensive exploration of the effects of pressure and temperature beyond conditions accessible in the laboratory. Our results show a distinct relationship between an alloying element's size and its diffusivity, which can be used to predict these properties for other elements not studied here. Differences in diffusivity mean that certain elements equilibrated in core-forming liquids faster than others. Differences in atomic size mean that certain elements fit more easily into the liquid iron structure than others, which affects their solubility. Understanding the trends by which diffusion rates and atomic radii change with pressure and temperature allows accurate modeling of the Earth's core under extreme conditions.
