  The Earth's crust is known to be depleted of gold, among other slightly heavy
noble metals transported by magma from the Earth's mantle to the crust. The
bulk silicate Earth (BSE) model also suggests significant depletion of Au in
the silicate mantle itself, which cannot be explained by the amount of Au in
the mantle's magma. This implies that Au could remain in the lower mantle and
form stable compounds, especially with iron, which is the predominant element
within the core. While Fe does not form binary compounds or a bulk alloy with
Au under ambient conditions, it may do so at the elevated pressures found in
the Earth's interior. Here, using density-functional methods, we investigated
the possibility of identifying stable, binary Fe-Au compounds at pressures up
to 210 GPa. We found three such Fe-Au compounds, which are stabilized by
pressure and notable electron transfer, including an orthorhombic AuFe$_4$
phase that is ferromagnetic in nature with Au possessing a significant magnetic
moment. While our results suggest that thermal convection due to the
conductivities and the heat flux from the Fe-Au compounds could be an energy
source to power the Earth's geodynamo, they also point towards changes in Au's
chemical properties, as it can exist as either an anion or cation under
pressure. In addition, the sound velocity and the density predicted for the
various Fe-Au compounds suggest that they could shed light on the composition
of the core-mantle boundary and the Earth's core, while demonstrating how the
presence of trace amounts of Au could influence agreement with seismic data.

Materials Science