Light elements in Earth's core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron-electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m-1⋅K-1 for liquid Fe-9Si near the topmost outer core. If Earth's core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core-mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core-mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core.

Center for Advanced Radiation Sources University of Chicago Chicago IL 60637

Department of Applied Physics Nanjing University of Science and Technology Nanjing 210094 China

Department of Chemistry University of Illinois Chicago Chicago IL 60607

Department of Earth and Environmental Sciences University of Illinois Chicago Chicago IL 60607

Department of Geological Sciences Jackson School of Geosciences The University of Texas at Austin Austin TX 78712

Department of Physics University of Illinois Chicago Chicago IL 60607

Department of Physics University of Illinois Chicago Chicago IL 60607;

Earth and Planets Laboratory Carnegie Institution for Science Washington DC 20015

Earth and Planets Laboratory Carnegie Institution for Science Washington DC 20015;

Institute of Atomic and Molecular Physics Sichuan University Chengdu 610065 China

New Technologies Research Centre University of West Bohemia 306 14 Plzeň Czech Republic

State Key Laboratory of Geodesy and Earth's Dynamics Innovation Academy for Precision Measurement Science and Technology Chinese Academy of Sciences Wuhan 430077 China

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