Sound velocity and elastic properties of Fe–Ni–S–Si liquid: the effects of pressure and multiple light elements

Fe–Ni–S–Si alloy is considered to be one of the plausible candidates of Mercury core material. Elastic properties of Fe–Ni–S–Si liquid are important to reveal the density profile of the Mercury core. In this study, we measured the P-wave velocity (VP) of Fe–Ni–S–Si (Fe73Ni10S10Si7, Fe72Ni10S5Si13, and Fe67Ni10S10Si13) liquids up to 17 GPa and 2000 K to study the effects of pressure, temperature, and multiple light elements (S and Si) on the VP and elastic properties. The VP of Fe–Ni–S–Si liquids are less sensitive to temperature. The effect of pressure on the VP are close to that of liquid Fe and smaller than those of Fe–Ni–S and Fe–Ni–Si liquids. Obtained elastic properties are KS0 = 99.1(9.4) GPa, KS’ = 3.8(0.1) and ρ0 =6.48 g/cm3 for S-rich Fe73Ni10S10Si7 liquid and KS0 = 112.1(1.5) GPa, KS’ = 4.0(0.1) and ρ0=6.64 g/cm3 for Si-rich Fe72Ni10S5Si13 liquid. The VP of Fe–Ni–S–Si liquids locate in between those of Fe–Ni–S and Fe–Ni–Si liquids. This suggests that the effect of multiple light element (S and Si) on the VP is suppressed and cancel out the effects of single light elements (S and Si) on the VP. The effect of composition on the EOS in the Fe–Ni–S–Si system is indispensable to estimate the core composition combined with the geodesy data of upcoming Mercury mission