Sound Velocity of Liquid Fe-P at High Pressure

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 K-S0 = 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%.