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

- The EoS of γ-Fe is constructed from the P-V-T data obtained at 0-24 GPa and 873-1873 K.
- The spin transition of γ-Fe from a mixed- or intermediate-spin state to low-spin state may be subtle.
- The EoS gives reference densities of metallic cores in planetary bodies.

In-situ synchrotron X-ray diffraction experiments using a multi-anvil apparatus have been conducted on face-centered cubic iron (γ-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 γ-Fe. A fit with a high-temperature Birch-Murnaghan (HT-BM) EoS yields the unit-cell volume V0, 1273 K=49.026(25) Å3, isothermal bulk modulus K1273 K=110.8(18) GPa, pressure derivative of the bulk modulus K′=5.3(2), temperature derivative of the bulk modulus (∂KT/∂T)P=−0.0288(17) GPa K−1 and thermal expansion coefficient α=4.50(36)×10−5+1.81(30)×10−8 T(K) K−1, respectively, at 0 GPa and 1273 K. A fit of the Mie-Grüneisen-Debye BM-EoS yields V0, 1273 K=49.026(25) Å3, K1273 K=111.5(18) GPa, K′=5.2(2), the Grüneisen parameter γ0=2.28(4), and a dimensionless parameter q=−0.21(22), with the fixed Debye temperature θ0=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 γ-Fe is relatively small. The newly determined EoS of γ-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.