Equation of state of γ-Fe: Reference density for planetary cores

In-situ synchrotron X-ray diffraction experiments using a multi-anvil apparatus have been conducted on face-centered cubic iron (gamma-Fe), which is a possible component of metallic cores in planetary bodies. From the pressure-volume-temperature (P-V-T) data collected systematically at 0-24 GPa and 873-1873 K, we have constructed the thermal equation of state (EoS) of gamma-Fe. A fit with a high-temperature BirchMumaghan (HT-BM) EoS yields the unit-cell volume V-0,V- (1273 k)=49.026(25) angstrom(3), isothermal bulk modulus K-1273 k=110.8(18) GPa, pressure derivative of the bulk modulus K'=5.3(2), temperature derivative of the bulk modulus (partial derivative K-T/partial derivative(T))(p)=-0.0288(17) GPa K-1 and thermal expansion coefficient alpha=4.50(36) x 10(-5)+1.81 (30) x 10(-8) T(K) K-1, respectively, at 0 GPa and 1273 K. A fit of the Mie-Gruneisen-Debye BM-EoS yields Vo, 1273 k=49.026(25) K-1273 (k)=111.5(18) GPa, K'=5.2(2), the Gruneisen parameter gamma(0)=2.28(4), and a dimensionless parameter q=-0.21(22), with the fixed Debye temperature theta(0)=340 K. From the present P-V-T data, the spin transformation from mixed- or intermediate-spin to low-spin states is considered to occur with increasing pressure. The successive transition may be subtle, however and its effect on the compression behavior of gamma-Fe is relatively small. The newly determined EoS of gamma-Fe enables us to more precisely estimate the density of the metallic cores of Mercury, Mars and planetary satellites such as the Moon and Ganymede. The estimated densities of cores in those planetary bodies depend strongly on adapted temperatures in the range of those previously proposed. The metallic cores of those planets as well as that of Earth might contain some other elements such as Ni, S, C, Si, O, and H. The core densities determined in this study provide a reference point to discuss the thermal and compositional structures in planetary and satellite cores when their core densities are determined by future surveys. (C) 2013 Elsevier B.V. All rights reserved.