Density measurements of liquid Fe-Si alloys at high pressure using the sink-float method

The compositional dependence on the density of liquid Fe alloys under high pressure is important for estimating the amount of light elements in the Earth's outer core. Here, we report on the density of liquid Fe-Si at 4 GPa and 1,923 K measured using the sink-float method and our investigation on the effect of the Si content on the density of the liquid. Our experiments show that the density of liquid Fe-Si decreases from 7.43 to 2.71 g/cm³ non-linearly with increasing Si content (0-100 at%). The molar volume of liquid Fe-Si calculated from the measured density gradually decreases in the compositional range 0-50 at% Si, and increases in the range 50-100 at% Si. It should be noted that the estimated molar volume of the alloys shows a negative volume of mixing between Fe and Si. This behaviour is similar to Fe-S liquid (Nishida et al. in Phys Chem Miner 35:417-423, 2008). However, the excess molar volume of mixing for the liquid Fe-Si is smaller than that of liquid Fe-S. The light element contents in the outer core estimated previously may be an underestimation if we take into account the possible negative value of the excess mixing volume of iron-light element alloys in the outer core.