Sound Velocity of Liquid Fe–P at High Pressure

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The longitudinal sound wave (P‐wave) velocity of liquid Fe75P25 alloy is determined up to 60 GPa using inelastic X‐ray scattering with laser‐heated diamond anvil cell. Phosphorus is found to have negligible effect on the P‐wave velocity of liquid Fe under high‐pressure conditions. Based on the P‐wave and pressure data, an adiabatic Murnaghan equation of state for liquid Fe75P25 is constructed, yielding to the adiabatic bulk modulus KS0 = 94.9 ± 7.1 GPa and its pressure derivative K’S = 3.56 ± 0.18, respectively, at 1 bar with the density of 5.94 g cm−3 at 2000 K. The addition of phosphorus decreases the bulk modulus and density of liquid Fe under high‐pressure conditions, which minimizes the effect on the velocity. Using the equation of state, the P‐wave velocity and density of liquid Fe75P25 are estimated under Earth's liquid outer core conditions. Comparison with seismological observations suggests an upper limit of phosphorus in liquid core of 14.6 ± 3.9 wt%.