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

<jats:p>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. A combined fit to all recent compression data up to 300 GPa gives the following Birch‐Murnaghan equation‐of‐state (EOS) parameters for iron: <jats:italic>V</jats:italic><jats:sub>02</jats:sub> = 6.73(1) cm<jats:sup>3</jats:sup> mol<jats:sup>−1</jats:sup>, <jats:italic>K</jats:italic><jats:sub>02</jats:sub> = 165(4) GPa, and <jats:italic>K</jats:italic>′<jats:sub>02</jats:sub> = 5.33(9). (Value in parentheses refers to the uncertainty of the last digit; e.g., 6.73(1) refers to 6.73+0.01.). Similar parameters are obtained with a recent “universal” form of the EOS of solids. For an Fe<jats:sub>0.8</jats:sub> Ni<jats:sub>0.2</jats:sub> alloy, the equation‐of‐state parameters are nearly identical, within error: <jats:italic>V</jats:italic><jats:sub>02</jats:sub> = 6.737(5) cm<jats:sup>3</jats:sup> mol<jats:sup>−1</jats:sup>, <jats:italic>K</jats:italic><jats:sub>02</jats:sub> = 172(2) GPa, and <jats:italic>K</jats:italic>′<jats:sub>02</jats:sub> = 4.95(9). In terms of volume, the alloy equation‐of‐state is indistinguishable from that of pure iron and the densities differ (dominantly in proportion to their atomic weights) by ∼0.3 Mg m<jats:sup>−3</jats:sup> at 330 GPa. 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.</jats:p>