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Register a new userFe-C and Fe-H systems at pressures of the Earths inner core
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The solid inner core of the Earth is predominantly composed of iron alloyed with several percent Ni and some lighter elements, Si, S, O, H, and C being the prime candidates. There have been a growing number of papers investigating C and H as possible light elements in the core, but the results are contradictory. Here, using ab initio simulations, we study the Fe-C and Fe-H systems at inner core pressures (330-364 GPa). Using the evolutionary structure prediction algorithm USPEX, we have determined the lowest-enthalpy structures of possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4 and Fe7C3) and hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C (Pnma) is the most stable iron carbide at pressures of the inner core, while FeH, FeH3 and FeH4 are stable iron hydrides at these conditions. For Fe3C, the cementite structure (Pnma) and the Cmcm structure recently found by random sampling are less stable than the I-4 and C2/m structures found here. We found that FeH3 and FeH4 adopt chemically interesting thermodynamically stable structures, in both compounds containing trivalent iron. The density of the inner core can be matched with a reasonable concentration of carbon, 11-15 mol.percent (2.6-3.7 wt.percent) at relevant pressures and temperatures. This concentration matches that in CI carbonaceous chondrites and corresponds to the average atomic mass in the range 49.3-51.0, in close agreement with inferences from the Birchs law for the inner core. Similarly made estimates for the maximum hydrogen content are unrealistically high, 17-22 mol.percent (0.4-0.5 wt.percent), which corresponds to the average atomic mass in the range 43.8-46.5. We conclude that carbon is a better candidate light alloying element than hydrogen.
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Fe, Mg, and O are among the most abundant elements in terrestrial planets. While the behavior of the Fe-O, Mg-O, and Fe-Mg binary systems under pressure have been investigated, there are still very few studies of the Fe-Mg-O ternary system at relevant Earths core and super-Earths mantle pressures. Here, we use the adaptive genetic algorithm (AGA) to study ternary Fe$_x$Mg$_y$O$_z$ phases in a wide range of stoichiometries at 200 GPa and 350 GPa. We discovered three dynamically stable phases with stoichiometries FeMg$_2$O$_4$, Fe$_2$MgO$_4$, and FeMg$_3$O$_4$ with lower enthalpy than any known combination of Fe-Mg-O high-pressure compounds at 350 GPa. With the discovery of these phases, we construct the Fe-Mg-O ternary convex hull. We further clarify the composition- and pressure-dependence of structural motifs with the analysis of the AGA-found stable and metastable structures. Analysis of binary and ternary stable phases suggest that O, Mg, or both could stabilize a BCC iron alloy at inner core pressures.
We present a new technique which allows the fully {em ab initio} calculation of the chemical potential of a substitutional impurity in a high-temperature crystal, including harmonic and anharmonic lattice vibrations. The technique uses the combination of thermodynamic integration and reference models developed recently for the {em ab initio} calculation of the free energy of liquids and anharmonic solids. We apply the technique to the case of the substitutional oxygen impurity in h.c.p. iron under Earths core conditions, which earlier static {em ab initio} calculations indicated to be thermodynamically very unstable. Our results show that entropic effects arising from the large vibrational amplitude of the oxygen impurity give a major reduction of the oxygen chemical potential, so that oxygen dissolved in h.c.p. iron may be stabilised at concentrations up a few mol % under core conditions.
We have studied the body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp) phases of Fe alloyed with 25 at. % of Ni at Earths core conditions using an ab initio local density approximation + dynamical mean-field theory (LDA+DMFT) approach. The alloys have been modeled by ordered crystal structures based on the bcc, fcc, and hcp unit cells with minimum possible cell size allowing for the proper composition. Our calculations demonstrate that the strength of electronic correlations on the Fe 3d shell is highly sensitive to the phase and local environment. In the bcc phase the 3d electrons at the Fe site with Fe only nearest neighbors remain rather strongly correlated even at extreme pressure-temperature conditions, with the local and uniform magnetic susceptibility exhibiting a Curie-Weiss-like temperature evolution and the quasi-particle lifetime {Gamma} featuring a non-Fermi-liquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase we predict a weakly-correlated Fermi-liquid state with a temperature-independent local susceptibility and a quadratic temperature dependence of {Gamma}. The iron sites with nickel atoms in the local environment exhibit behavior in the range between those two extreme cases, with the strength of correlations gradually increasing along the hcp-fcc-bcc sequence. Further, the inter-site magnetic interactions in the bcc and hcp phases are also strongly affected by the presence of Ni nearest neighbors. The sensitivity to the local environment is related to modifications of the Fe partial density of states due to mixing with Ni 3d-states.
This work extends our Object Kinetic Monte Carlo model for neutron irradiation-induced nanostructure evolution in Fe-C alloys to consider higher irradiation temperatures. The previous study concentrated on irradiation temperatures < 370 K. Here we study the evolution of vacancy and self-interstitial atom (SIA) cluster populations at the operational temperature of light water reactors, by simulating specific reference irradiation experiments. Following our previous study, the effect of carbon on radiation defect evolution can be described in terms of formation of immobile complexes with vacancies, that in turn act as traps for SIA clusters. This dynamics is simulated using generic traps for SIA and vacancy clusters. The traps have a binding energy that depends on the size and type of the clusters and is also chosen on the basis of previously performed atomistic studies. The model had to be adapted to account for the existence of two kinds of SIA clusters, <111> and <100>, as observed in electron microscopy examinations of Fe alloys neutron irradiated at the temperatures of technological interest. The model, which is fully based on physical considerations and only uses a few parameters for calibration, is found to be capable of reproducing the experimental trends, thereby providing insight into the physical mechanisms of importance to determine the type of nanostructural evolution undergone by the material during irradiation.
This paper continues our previous work on a nanostructural evolution model for Fe-C alloys under irradiation, using Object Kinetic Monte Carlo modeling techniques. We here present a number of sensitivity studies of parameters of the model, such as the carbon content in the material, represented by generic traps for point defects, the importance of traps, the size dependence of traps and the effect of the dose rate.
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