Static compression of iron to 300 GPa and Fe_0.8Ni_0.2 alloy to 260 GPa: implications for composition of the core

We report the results of X-ray diffraction experiments with the diamond anvil cell to pressures above 300 GPa at room temperature on pure iron and an iron-nickel alloy. These data extend throughout the pressure range of the bulk of the outer core of the Earth and provide for the first time direct pressure-volume measurements on geophysically important materials at such conditions. Both iron and iron-nickel are observed to remain in the hexagonal close-packed structure to the maximum pressures. Within the range of uncertainty in Earth model densities and trade-offs with the percentage light component in the core, nickel could be present in the core in an amount at least equal to its estimated abundance in the Earth. A direct comparison with (solid) inner core densities is now possible and places direct constraints on the thermal models of the Earth's interior. -from Authors