Structure Stability and Compressibility of Fe2Si at High Pressure

Silicon (Si) has been considered as one of the principle light elements in the Earth's iron dominated core that can account for the density deficit and velocity discrepancy between pure iron and seismic values (Birch, 1952). Recent studies indicated the core could contain up to 6-8 wt.% Si (Fischer et al., 2014; Ozawa et al., 2016). To understand the effect of silicon on the elastic properties of iron in the Fe-rich core, knowledge on elastic properties of Fe-Si alloys need to be extended to core conditions. Unlike the extensively studied FeSi with high pressure data for different structures, Fe₂Si was only found in a lunar fragmental breccia meteorite (Anand et al., 2002) and its high pressure property has barely been studied. In this study, we conducted synchrotron X-ray diffraction experiments on Fe₂Si at high pressures in a diamond anvil cell (DAC) and room temperature condition. A third-order Birch-Murnaghan equation of state fit to our XRD data and yields equation of state parameters that is comparable to previous theoretical calculation. The closest well-studied member in Fe-FeSi system to Fe₂Si is Fe-17Si. Bulk modulus for Fe-17Si is larger than Fe₂Si, as indicated by our experimental data. This is consistent with previous result that increase in Si content in the Fe-Si alloy will decrease the bulk modulus with similar crystal structures (Fischer et al., 2014). Further, based on statistical methods for the equation of state fittings, the correlation between K₀ and K₀' will be discussed. Our results would provide new insights into the understanding of Fe-Si system in the Earth's core and small rocky planets such as Mercury and Mars.

References:

Anand M. et al., 33^rd Annual Lunar and Planetary Science Conference, 2002: no.1653

Birch F., Journal of Geophysical Research, 1952, 57(2): 227-286

Fischer R. et al., Journal of Geophysical Research: Solid Earth, 2014,119(4):2810-2827.

Ozawa H. et al., Earth and Planetary Science Letters, 2016, 456(15): 47-54