Carbon is a plausible light element candidate in the Earth's outer core. We measured the density of liquid Fe-3.5wt% C up to 6.8GPa and 2200K using an X-ray absorption method. The compression curve of liquid Fe-C was fitted using the third-order Birch-Murnaghan equation of state. The bulk modulus and its pressure derivative are K0,1500K=55.3±2.5GPa and (dK0/dP)T=5.2±1.5, and the thermal expansion coefficient is α=0.86±0.04×10-4K-1. The Fe-C density abruptly increases at pressures between 4.3 and 5.5GPa in the range of present temperatures. Compared with the results of previous density measurements of liquid Fe-C, the effect of carbon on the density of liquid Fe shows a nonideal mixing behavior. The abrupt density increase and nonideal mixing behavior are important factors in determining the light element content in the Earth's core.
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