The distributions of major and minor elements in Earth’s core and mantle were primarily established by high pressure, high temperature metal–silicate partitioning during core segregation. The partitioning behaviors of moderately siderophile elements can be used to constrain the pressure–temperature conditions of core formation and the core’s composition. We performed experiments to study the partitioning of Ni, Co, V, Cr, Si, and O between silicate melt and Fe-rich metallic melt in a multianvil press and diamond anvil cell, up to 100GPa and 5700K. Combining our new results with data from 18 previous studies, we parameterized the effects of pressure, temperature, and metallic melt composition on partitioning. Ni and Co partitioning are insensitive to composition. At low pressures, these elements become less siderophile with increasing temperature, with this trend reversing above ∼45GPa. V and Cr partitioning are much more sensitive to metallic melt composition and less sensitive to pressure. Partitioning of Si and O are insensitive to pressure, but with strong and moderate temperature dependences, respectively. Our new parameterizations of Ni and Co partitioning suggest that the Earth’s distributions of these elements can be matched by single-stage core–mantle equilibration at 54±5GPa and 3300–3400K. These conditions would result in 8.5±1.4wt% Si and 1.6±0.3wt% O in the core, compatible with the core’s measured density. However, this single-stage model matches the Earth’s V and Cr distributions less well. We also incorporated our parameterizations into models of multi-stage core formation over evolving pressure–temperature-oxygen fugacity conditions, reproducing the Earth’s Ni and Co distributions while simultaneously producing a core whose light element composition is consistent with its density

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride ${\mathrm{FeB}}_{4}$ were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in ${\mathrm{FeB}}_{4}$ is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

The light alloying element in the Earth's core has not been identified yet. Here we determined the pressure‐volume equations of state of FeSi, Fe₃C, and Fe_0.95O in the core pressure range by a combination of diamond‐anvil cell and synchrotron X‐ray diffraction techniques. Both B2‐type FeSi and Fe₃C cementite were preserved to 180 and 187 GPa, respectively. The rhombohedrally‐distorted B1 phase of Fe_0.95O was measured up to 186 GPa, and the distorted B8‐type Fe_0.95O was observed between 170 and 226 GPa. Combined with our previous data on FeS VI and B2‐type VII phases to 270 GPa, we discuss the light element in the outer core by comparing the densities and compressibilities of these iron compounds with seismologically‐estimated density profile in the core. Substitution of light element, particularly carbon and oxygen, in iron not only reduces the density but also enhances the compressibility remarkably. The core profile is therefore not reconciled with Fe‐C and Fe‐O compounds, while the densities and compressibilities of Fe‐Si and Fe‐S alloys match the observations. Carbon and oxygen may not be a predominant light element in the Earth's outer core, leaving silicon and sulfur as strong candidates.