Thermal expansion of iron-rich alloys and implications for the Earth's core

Abstract

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 = a ₁ + a ₂ T, where a ₁ = 3.0 ± 1.3 × 10⁻⁵ K⁻¹ and a ₂ = 2.8 ± 1.5 × 10⁻⁸ K⁻². The temperature dependence of isothermal bulk modulus (∂K _T,0/∂T)_P is estimated at −3.75 ± 1.80 × 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 Fe_0.864Si_0.136 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.

• Fe3S
• Fe–Si alloy
• light element