  First principles electronic structure calculations based on density
functional theory have been used to study the thermodynamic, structural and
transport properties of solid 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 account for the inferred density in the outer core, to
probe the stability of the liquid against phase separation, to interpret the
bonding in the liquid, and to find out whether the viscosity differs
significantly from that of pure liquid iron at the same conditions. It is shown
that the required concentration of oxygen is in the region 25-30 mol percent,
and evidence is presented for phase stability at these conditions. The Fe-O
bonding is partly ionic, but with a strong covalent component. The viscosity is
lower than that of pure liquid iron at Earth's core conditions. It is shown
that earlier first-principles calculations indicating very large enthalpies of
formation of solid solutions may need reinterpretation, since the assumed
crystal structures are not the most stable at the oxygen concentration of
interest.
