Thermal Expansion of Iron-Rich Alloys and Implications for the Earth's Core

Understanding the thermal-chemical state of the Earth's core requires knowledge of the thermal expansion of iron-rich alloys at megabar pressures and high temperatures. Our survey of literature revealed a significant lack of such data. We have determined the unit-cell parameters of the iron-sulfur compound Fe3S by using synchrotron x-ray diffraction techniques and externally heated diamond-anvil cells at pressures up to 42.5 GPa and temperatures up to 900 K. The zero-pressure thermal expansivity of Fe3S is determined in the form α = a1 + a2 T, where a1 = 3.0 ± 1.3 x 10-5 K-1 and α2 = 2.8 ± 1.5 x 10-8 K-2. The temperature dependence of isothermal bulk modulus (∂KT,0/∂T)P is estimated at -3.75 ± 1.80 x 10-2 GPa K-1. Our data at 42.5 GPa and 900 K suggest that ∼2.1 at. % (1.2 wt. %) sulfur produces 1% density deficit in iron. We have also carried out energy-dispersive x-ray diffraction measurements on pure iron and Fe0.864Si0.136 alloy samples that were placed symmetrically in the same multianvil cell assemblies, using the SPring-8 synchrotron facility in Japan. Based on direct comparison of unit cell volumes under presumably identical pressures and temperatures, our data suggest that at most 3.2 at. % (1.6 wt. %) silicon is needed to produce 1% density deficit with respect to pure iron