The sound velocity (
V P) of liquid Fe–10 wt% Ni and Fe–10 wt% Ni–4 wt% C up to 6.6 GPa was studied using the ultrasonic pulse-echo method combined with synchrotron X-ray techniques. The obtained
V P of liquid Fe–Ni is insensitive to temperature, whereas that of liquid Fe–Ni–C tends to decrease with increasing temperature. The
V P values of both liquid Fe–Ni and Fe–Ni–C increase with pressure. Alloying with 10 wt% of Ni slightly reduces the
V P of liquid Fe, whereas alloying with C is likely to increase the
V P. However, a difference in
V P between liquid Fe–Ni and Fe–Ni–C becomes to be smaller at higher temperature. By fitting the measured
V P data with the Murnaghan equation of state, the adiabatic bulk modulus (
K S0) and its pressure derivative (
K S ′ ) were obtained to be
K S0 = 103 GPa and
K S ′ = 5.7 for liquid Fe–Ni and
K S0 = 110 GPa and
K S ′ = 7.6 for liquid Fe–Ni–C. The calculated density of liquid Fe–Ni–C using the obtained elastic parameters was consistent with the density values measured directly using the X-ray computed tomography technique. In the relation between the density (
ρ) and sound velocity (
V P) at 5 GPa (the lunar core condition), it was found that the effect of alloying Fe with Ni was that
ρ increased mildly and
V P decreased, whereas the effect of C dissolution was to decrease
ρ but increase
V P. In contrast, alloying with S significantly reduces both
ρ and
V P. Therefore, the effects of light elements (C and S) and Ni on the
ρ and
V P of liquid Fe are quite different under the lunar core conditions, providing a clue to constrain the light element in the lunar core by comparing with lunar seismic data.
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