Oxygen in the Earth's core: a first-principles study

First-principles electronic structure calculations based on DFT have been u sed to study the thermodynamic, structural and transport properties of soli d solutions and liquid alloys of iron and oxygen at Earth's core conditions . Aims of the work are to determine the oxygen concentration needed to acco unt for the inferred density in the outer core, to probe the stability of t he liquid against phase separation, to interpret the bonding in the liquid, and to find out whether the viscosity differs significantly from that of p ure liquid iron at the same conditions. It is shown that the required conce ntration of oxygen is in the region 25-30 mol%, and evidence is presented f or phase stability at these conditions. The Fe/O bonding is partly ionic, b ut with a strong covalent component. The viscosity is lower than that of pu re liquid iron at Earth's core conditions. It is shown that earlier first-p rinciples calculations indicating very large enthalpies of formation of sol id solutions may need reinterpretation, since the assumed crystal structure s are not the most stable at the oxygen concentration of interest. (C) 1999 Elsevier Science B.V. All rights reserved.