Static Compression of an Iron-Silicon Alloy: Implications for Silicon in the Core

J F Lin, A J Campbell, D L Heinz1, J M Devine, and G Shen2 (Department of the Geophysical Sciences, 1and James Franck Institute, 2and Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637; afu@geosci.uchicago.edu)

In situ energy-dispersive X-ray diffraction experiments of Fe8wt\%Si at room temperature and high pressures up to 54 GPa revealed that a phase transformation from bcc to hcp began at 16 GPa and was completed by 36 GPa. The isothermal bulk modulus of the hcp phase is 141 (10) GPa, its pressure derivative is 5.70 (0.60), and the zero-pressure volume is 6.882 (0.021) cm$^{3}$/mol. Fe8wt\%Si forms a solid solution with Fe and thus has a similar phase diagram. Upon comparison with the Preliminary Reference Earth Model, the extended equation of state and calculated bulk sound velocity of hcp-Fe8wt\%Si imply that the allowable Si in the outer core is about 4 wt\% unless other light elements have compensating elastic properties.