First-principles Simulations of Liquid Iron-heavy Element Alloys At High Pressure

Liquid iron is thought to contain many elements under Earths core conditions. While the incorporation of light elements has been studied extensively, our knowledge about liquid iron-heavy element mixtures is limited. Here we investigate the structural and dynamical behavior of four siderophile elements, namely Co, Ni, Mo, and W dissolved in liquid iron using first-principles molecular dynamics over wide ranges of pressure (up to 350 GPa) and temperature (4000 to 7000 K). The simulation results show that Mo and W increase the density much more than Co and Ni while the bulk modulus remains almost unaffected in all cases. To explain the density and seismic velocity profiles of the outer core, light elements like H and C must be added to liquid iron alloys in large amounts. The calculated mean iron coordination number of each of four heavy elements considered is somewhat larger than the mean Fe-Fe coordination number. This suggests that their incorporation is of substitutional nature unlike interstitial mechanism for H and C. The diffusion rates of these impurities are found to be comparable to that of host atoms, but they are much lower than those of light elements.