Hugoniot equation of state of cementite (Fe3C) up to 250 GPa and its geophysical implications

Dense bulk Fe3C samples were synthesized by the high pressure–temperature method using a large-volume cubic press. The Hugoniot data of the Fe3C samples were measured from 65 to 250 GPa on a two-stage light gas gun. The shock velocities were measured by the electrical pin technique, and the particle velocities were calculated by the impedance match method. The shock velocity (Us) and particle velocity (Up) relationship of Fe3C was determined to be linear and described by Us = 4.77(7) + 1.60(2) Up. The thermal contributions of electronics and lattices were determined by first-principle simulation and thermodynamic calculation, respectively. Subsequently, the thermal equation of state of Fe3C was used to constrain the carbon amount in the inner core. We conclude that 2.9 ± 0.4 wt% carbon is needed to compensate for the density deficit between pure iron and the inner core, if carbon is the sole light element present in the core.