Short- and Intermediate-Range Structure and Dynamics of Fe-Ni-C Liquid Under Compression

Jianwei Wang,Bin Chen,Quentin Williams,Murli H. Manghnani

doi:10.3389/feart.2019.00258

Abstract

Properties of liquid Fe alloys under high-pressure conditions are crucial for understanding the composition, thermal state, and dynamics of Earth’s core. Experiments on such liquids, however, are often performed under pressures far below those of the outer core, necessitating long extrapolations of experimental results to core conditions. Such estimates can be complicated by light elements possibly forming pressure-dependent molecular clusters that can significantly affect the physical properties of liquids as core conditions are approached. First-principles molecular dynamics simulations were employed to compute the properties of an Fe-Ni-C liquid with a composition of Fe3.7Ni0.37C at 1673 K and pressures from 0 to 67 GPa to benchmark computational methods on pressure effects on the structure and properties of the liquid relative to low pressure experimental results. The short-range structure is manifested by the coordination number (CN) of Fe/Ni-Fe/Ni being around 12, indicative of a nearly close-packed structure in the pressure range, and the CN of C-Fe/Ni gradual increasing from 6.5 to 8.5, indicative of an approximately octahedral to cubic transition as pressure increases. The Fe/Ni-Fe/Ni bond distance, however, is found to be 10 times more compressible than the C-Fe/Ni distance. The intermediate-range structure of Fe/Ni-Fe/Ni and C-Fe/Ni subsystems, described by a partial configurationally-decomposed distribution function, undergoes substantial changes, characterized by a significant increase of the number of polyhedra that share 3 atoms with each other. Such dense configurations are related to an increased bulk modulus, decreased diffusion coefficient, decreased activation volume for diffusion, and increased shear viscosity. Reproducing the experimental observations at low pressures provides important support for modeling the liquid under the conditions relevant to the outer core.

Highlights

Earth’s core is predominately Fe with ∼5–10% Ni (Birch, 1952), as inferred from seismic velocities, cosmic abundances of elements (McDonough, 2003), and the understanding of Earth’s accretion and differentiation processes (Hart and Zindler, 1986; Kleine et al, 2002; Rubie et al, 2015)
By considering the bulk composition of the Earth, the chemical affinities to Fe, and processes involving incorporation of light elements to the outer core with constraints on the core density, seismic wave velocity, and solubility in liquid Fe, there are a number of feasible candidate light elements (Birch, 1964; Wood, 1993; Poirier, 1994; Li and Fei, 2007; Wood et al, 2013; Badro et al, 2014, 2015), including H, C, O, S, and Si
The C atoms prefer to bond to Fe relative to Ni at all pressures studied, and the Fe and Ni atoms appear randomly mixed at low pressure, but Ni becomes more likely to bond to Fe than itself as pressure increases

Summary

Introduction

Earth’s core is predominately Fe with ∼5–10% Ni (Birch, 1952), as inferred from seismic velocities, cosmic abundances of elements (McDonough, 2003), and the understanding of Earth’s accretion and differentiation processes (Hart and Zindler, 1986; Kleine et al, 2002; Rubie et al, 2015). By considering the bulk composition of the Earth, the chemical affinities to Fe, and processes involving incorporation of light elements to the outer core with constraints on the core density, seismic wave velocity, and solubility in liquid Fe, there are a number of feasible candidate light elements (Birch, 1964; Wood, 1993; Poirier, 1994; Li and Fei, 2007; Wood et al, 2013; Badro et al, 2014, 2015), including H, C, O, S, and Si. solutions to which light elements might be dominant are not unique and depend on the methodology used to determine the most likely light elements given a set of constraints. The chemical bonding between C and Fe/Ni and the short- and intermediate-range structures of the candidate liquid alloys likely play an important role in determining the elastic and transport properties that are relevant to modeling of the Earth’s outer core

Methods

Results

Conclusion