Mass Transport and Structural Properties of Binary Liquid Iron Alloys at High Pressure

Abstract

We determine mass transport and structural properties of binary liquid iron alloys over a wide density (5.055–11.735 g·cm−3) and temperature range (2,500–6,500 K) using first‐principles molecular dynamics. Compositions consist of 96 at% Fe and 4 at% ϕ, where ϕ = H, C, N, O, Mg, Si, S, or Ni. Self‐diffusion coefficients (D) of Fe and ϕ range from 3.5·10−9 to 1.9·10−7 m2·s−1. Results show a relation between mean atomic radius and diffusivity ratio for the alloying element and iron: Si and Ni are “iron‐like” with similar atomic radii and D compared with those of Fe; H, C, N, O, and S are “small non‐iron‐like” with smaller atomic radii and larger D; and Mg transitions from “large non‐iron‐like” with a larger atomic radius and smaller D at low density to iron‐like under conditions of the Earth's core. The effect of pressure on D for C, N, and O is negligible for densities below ~8 g·cm−3, accompanied by an increase in average coordination numbers to ~6, and an increase in mean atomic radii. For densities above ~8 g·cm−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.

Strong Sequestration of Hydrogen Into the Earth's Core During Planetary Differentiation

“…This is due to the fact that the silicate melt structure undergoes radical atomic arrangement from four‐ to six‐fold Si‐O coordination (SI, Figure S6a): While the nearest neighbor O‐H distance represented by the principal peak position in g HO ( r ) (SI, Figure S6b) remains at virtually the same distance, the second neighbor O‐H distances are shifted to considerably lower r with increasing P , reflected in the decrease of with P . By contrast, the average interatomic distance between Fe and H in the metallic melts remains constant over the same P ‐range (SI, Figure S6d), similar to the results reported in a previous DFT‐MD study (Posner & Steinle‐Neumann, 2019). At higher P , differences are Δ r V/ = −16% (40 GPa, 4,000 K) and −12% (130 GPa, 4,000 K), and they contribute more significantly to G by a P Δ r V energy component due to large P : The P Δ r V term increases from −0.13 eV at 20 GPa and 2,500 K, to −0.54 eV at 40 GPa and 4,000 K, and −0.88 eV at 130 GPa and 4,000 K (Figures 2d – 2f; SI, Table S2).…”

Ab Initio Thermoelasticity of Liquid Iron-Nickel-Light Element Alloys

“…In particular, in the latter, empirical pressure corrections of 10 GPa and 8 GPa were adopted at the CMB and ICB respectively, though the optimized OC compositions are essentially sensitive to these corrections. Meanwhile, some studies have been performed throughout the whole OC P, T conditions for pure Fe [7,26], Fe-S [27], and Fe-H [28]. However, different formulations were employed to model their thermal equations of state, making a quantitative comparison of the reported thermoelasticity not easy.…”

Structures of Liquid Iron–Light‐Element Mixtures under High Pressure

“…These features align with the results of earlier studies. [ 17,21,22,23 ] In addition, g OO ( r ) and g SiSi ( r ) do not depend on the LE concentration. For g FeFe ( r ), there were no differences in all systems.…”

“…[ 19 ] Theoretically, the structures of liquid FeH, FeC, FeN, FeO, FeMg, FeSi, and FeS under high pressure were investigated by ab initio molecular dynamics simulations. [ 20–23 ] Alfè and Gillan investigated the structure of liquid FeS at 6000 K and 330 GPa and suggested that there was no tendency for S atoms to form chains, despite the fact that pure sulfur is known to form chains in the liquid phase. [ 20 ] The structural properties of liquid FeO under high pressure were also investigated by ab initio MD simulations.…”

“…[ 22 ] From the peak positions of radial distribution functions obtained from these simulations, it was discovered that Si was incorporated in the liquid state in substitution with Fe atoms, even at 330 GPa. Recently, Posner and Steinle‐Neumann also performed first‐principles MD for liquid FeX (X = H, C, N, O, Mg, Si, S, and Ni with 4 at%), [ 23 ] investigating diffusion coefficients and structural properties. The results showed that Si and Ni are “iron‐like” elements, whereas H, C, N, O, and S are “small non‐iron‐like” elements.…”