Liquid-Liquid Transition in Fe-Ni-C alloy at High Temperature and Pressures Revealed by Structure and Viscosity from First-Principles Molecular Dynamics

Understanding and modeling of planetary core processes such as the geodynamo and heat flow via convection in the outer core require knowledge on the properties such as density and viscosity of candidate liquid iron alloys. Certain light elements are found to have significant effects on the properties of liquid iron due to the structural control of light elements and formation of large molecular units. However, most experiments are performed at pressures far below or about tenth of the outer core pressures, prompting long extrapolation of the experimental results to the core conditions. Such an extrapolation could become problematic with fraught uncertainty because of transitions in the liquid structure and large molecular unit formation at pressures out of reach of experimental and extrapolation methods. Thus, it is essential to use well-benchmarked theoretical methods such as first-principles molecular dynamics to compute the physical properties of the liquids at the outer core conditions. In this contribution, plane-wave density functional theory with PAW and spin polarized GGA and the PBE potentials were employed in molecular dynamics simulations to calculate the density, equation of state, magnetic property, radial distribution function, bonding and coordination environment, and viscosity of Fe100Ni105wt%C liquid alloy. The computational methods were benchmarked based on available experimental data on density, bulk modulus and its derivative, radial distribution function, and viscosity. The calculations confirm a liquid-liquid-phase transition of the alloy at 5GPa revealed by X-ray diffraction measurements, accompanied by a coordination number increase of from 11.7 to 12.2 of Fe/Ni to Fe/Ni and bond distance decrease of 18% Fi/Ni-C around 5GPa. The calculated viscosity compares favorability with experimental observations. The benchmarking leads to improved DFT calculations and a greater understanding of physical processes of the alloy and the controls of atomistic scale structure and dynamics on the calculated properties. Such understanding provides invaluable inputs in optimizing the first-principles molecular dynamics simulations. Based on well-benchmarked methods, the liquid properties were calculated to the core pressures.