Thermal Properties of Liquid Iron at Conditions of Planetary Cores

Thermal properties of iron at high pressures (P) and temperatures (T) are essential for determining the internal structure and evolution of planetary cores. Compared to its solid counterpart, the liquid phase of iron is less studied and existing results exhibit large discrepancies, hindering a proper understanding of planetary cores. Here we use the formally exact urn:x-wiley:21699097:media:jgre21861:jgre21861-math-0019 thermodynamic integration approach to calculate thermal properties of liquid iron up to 3.0 TPa and 25000 K. Uncertainties associated with theory are compensated by introducing a T-independent pressure shift based on experimental data. The resulting thermal equation of state agrees well with the diamond anvil cell (DAC) data in the P-T range of measurements. At higher P-T it matches the reduced shock wave data yet deviates considerably from the extrapolations of DAC measurements, indicating the latter may require further examinations. Moreover, the calculated heat capacity and thermal expansivity are substantially lower than some recent reports, which have important ramifications for understanding thermal evolutions of planetary cores. Using Kepler-36b as a prototype, we examine how a completely molten core may affect the P-T profiles of massive exoplanets. By comparing the melting slope and the adiabatic slope along the iron melting line, we propose that crystallization of the cores of massive planets proceeds from the bottom-up rather than the top-down

The phase diagram of NiSi under the conditions of small planetary interiors

The phase diagram of NiSi has been determined using in situ synchrotron X-ray powder diffraction multi-anvil experiments to 19 GPa, with further preliminary results in the laser-heated diamond cell reported to 60 GPa. The low-pressure MnP-structured phase transforms to two different high-pressure phases depending on the temperature: the ε-FeSi structure is stable at temperatures above ∼1100 K and a previously reported distorted-CuTi structure (with Pmmn symmetry) is stable at lower temperature. The invariant point is located at 12.8 ± 0.2 GPa and 1100 ± 20 K. At higher pressures, ε -FeSi-structured NiSi transforms to the CsCl structure with CsCl-NiSi as the liquidus phase above 30 GPa. The Clapeyron slope of this transition is -67 MPa/K. The phase boundary between the ε -FeSi and Pmmn structured phases is nearly pressure independent implying there will be a second sub-solidus invariant point between CsCl, ε -FeSi and Pmmn structures at higher pressure than attained in this study. In addition to these stable phases, the MnP structure was observed to spontaneously transform at room temperature to a new orthorhombic structure (also with Pnma symmetry) which had been detailed in previous ab initio simulations. This new phase of NiSi is shown here to be metastable

Equation of State of hcp Fe-C-Si Alloys and the Effect of C Incorporation Mechanism on the Density of hcp Fe alloys at 300 K 2

International audienceSi and C are cosmochemically abundant elements soluble in hcp Fe under pressure and temperature, and could therefore be present in the Earth’s inner core. While recent ab initio calculations suggest that the observed inner core density and velocities could be matched by an Fe-C-Si alloy, the combined effect of these two elements has only recently started to be investigated experimentally. We therefore carried out synchrotron X-ray diffraction measurements of an hcp Fe-C-Si alloy with 4 at% C and 3 at% Si, up to ∼150 GPa. Density functional theory calculations were also performed to examine different incorporation mechanisms. These calculations suggest interstitial C to be more stable than substitutional C below ~350 GPa. In our calculations, we also find that the lowest energy incorporation mechanism in the investigated pressure range (60-400 GPa) is one where two C atoms occupy one atomic site; however this is unlikely to be stable at high temperatures. Notably, substitutional C is observed to decrease the volume of the hcp Fe, while interstitial C increases it. This allowsus to use experimental and theoretical equations-of-state to show unambiguously that C in the experimental hcp Fe-C-Si alloys is not substitutional, as is often assumed. This is crucial since assuming an incorrect incorporation mechanism in experiments, leads to incorrect density determinations of ~4%, undermining attempts to estimate the concentration of C in the inner core. In addition, the agreement between our experiments and calculations, support Si and C as being light elements in the inner core