The relative abundance of light elements in the Earth's core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70 GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4–5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core.

We perform first-principles calculations to investigate liquid iron-sulfur alloys (Fe, Fe 56 S 8 , Fe 52 S 12 , and Fe 48 S 16 ) under high-pressure and high-temperature (150-300 GPa and 4000-6000 K) conditions corresponding to the Earth's outer core. Considering only the density profile, the best match with the preliminary reference Earth model is by liquid Fe-14 wt % S (Fe 50 S 14 ), assuming sulfur is the only light element. However, its bulk sound velocity is too high, in particular in the deep outer core, suggesting that another light component such as oxygen is required. An experimental check using inelastic X-ray scattering shows good agreement with the calculations. In addition, a present study demonstrates that the Birch's law does not hold for liquid iron-sulfur alloy, consistent with a previous report on pure liquid iron.

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The pressure response of lattice thermal conduction in MgO periclase has been a matter of interest for many decades to estimate the thermal conductivity profile of the lower mantle. Using the pulsed light heating thermoreflectance technique, we measured the lattice thermal diffusivity of MgO at pressures up to 137 GPa at 300 K to determine its lattice thermal conductivity under deep lower mantle conditions. Considering the temperature effect estimated by previous high-temperature measurements, we calculated the lattice part of the thermal conductivity of MgO to be 17.9 ± 1.1 W/m/K at 135 GPa and 3600 K. Additionally, we observed that the lattice conductivity of MgO has little dependence on its grain size under core-mantle boundary conditions.