Sound velocity and elastic properties of Fe–Ni and Fe–Ni–C liquids at high pressure
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- Kuwabara, S., Terasaki, H., Nishida, K. et al. Phys Chem Minerals (2016) 43: 229. doi:10.1007/s00269-015-0789-y
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The sound velocity (VP) 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 VP of liquid Fe–Ni is insensitive to temperature, whereas that of liquid Fe–Ni–C tends to decrease with increasing temperature. The VP values of both liquid Fe–Ni and Fe–Ni–C increase with pressure. Alloying with 10 wt% of Ni slightly reduces the VP of liquid Fe, whereas alloying with C is likely to increase the VP. However, a difference in VP between liquid Fe–Ni and Fe–Ni–C becomes to be smaller at higher temperature. By fitting the measured VP data with the Murnaghan equation of state, the adiabatic bulk modulus (KS0) and its pressure derivative (KS′) were obtained to be KS0 = 103 GPa and KS′ = 5.7 for liquid Fe–Ni and KS0 = 110 GPa and KS′ = 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 (VP) at 5 GPa (the lunar core condition), it was found that the effect of alloying Fe with Ni was that ρ increased mildly and VP decreased, whereas the effect of C dissolution was to decrease ρ but increase VP. In contrast, alloying with S significantly reduces both ρ and VP. Therefore, the effects of light elements (C and S) and Ni on the ρ and VP 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.