Fe–C and Fe–H systems at pressures of the Earth's inner core

Abstract

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. To establish the chemical composition of the inner core it is necessary to find the range of compositions that can explain its observed characteristics. Recently, 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, Fe 2 C, Fe 3 C, Fe 4 C, FeC 2 , FeC 3 , FeC 4 and Fe 7 C 3 ) and hydrides (Fe 4 H, Fe 3 H, Fe 2 H, FeH, FeH 2 , FeH 3 , FeH 4 ) and have found that Fe 2 C (space group Pnma) is the most stable iron carbide at pressures of the inner core, while FeH, FeH 3 and FeH 4 are stable iron hydrides at these conditions. For Fe 3 C, the cementite structure (space group Pnma) and the Cmcm structure recently found by random sampling are less stable than the I-4 and C2/m structures found here. We have found that FeH 3 and FeH 4 adopt chemically interesting thermodynamically stable structures, in both compounds containing trivalent iron. We find that the density of the inner core can be matched with a reasonable concentration of carbon, 11-15 mol % (2.6-3.7 wt. %) 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 Birch's law for the inner core. Similarly made estimates for the maximum hydrogen content are unrealistically high, 17-22 mol.% (0.4-0.5 wt. %), 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.

Synthesis of lithium polyhydrides above 130 GPa at 300 K

“…Measurements have been performed using the nonperturbing technique of synchrotron infrared absorption. The observed vibron frequencies match the predictions for LiH 2 and LiH 6 . These polyhydrides remain insulating up to 215 GPa.…”

“…The convex hull diagram of the relative enthalpies calculated for LiH n compound indicates that LiH should be always stable if the compressed sample keeps an equal ratio of Li and H (1). In the present case, the affinity of Li with carbon sucks the Li from the LiH sample at the interface with diamond, and the transformed interface layer evolves to the LiH n compound having the minimum enthalpy, predicted to be LiH 6 . The hypothesis of a Li/C chemical reaction was strengthened by GPa.…”

“…By going up in pressure, the appearance of LiH 6 and LiH 2 is, in fact, an illustration of the convexity of the hull diagram. Above 130 GPa, the diffusion of Li in C favors the formation of a LiH 6 layer in contact with the anvil.…”

First-principles calculations of properties of orthorhombic iron carbideFe7C3at the Earth's core conditions

“…Theoretical structure prediction calculations at core pressures and zero kelvin [16,17] have uncovered new Fe 3 C phases that are more stable than cementite at core pressures, and a stoichiometry which has not been previously discussed in relation to the Earth's core, Fe 2 C. These studies suggest that Fe 7 C 3 decomposes to Fe 3 C and Fe 2 C at core pressures, but there is a lack of experimental evidence to support their claims. Although experiments have ruled out the coexistence of pure iron and carbon at high pressures and temperature [18], ab initio calculations suggest that significant amounts (up to 6 at.%) of carbon can exist in crystalline phases of iron as interstitial defects under high pressure, suggesting the existence of an iron-carbon alloy [19].…”

“…Taking the effects of temperature into account, vibrational entropy stabilizes o-Fe 7 C 3 with respect to the hexagonal phase at 150 GPa, and by extrapolation, becomes more stable in excess of 1000 K. However, at the pressure of the Earth's core (360 GPa), o-Fe 7 C 3 is no longer competitive in our theoretical calculations. The possibility remains that Fe 7 C 3 may decompose to more stable stoichiometries such as Fe 2 C and Fe 3 C, as suggested by zero temperature structure prediction calculations [17]. However, confirmation would require many more expensive free energy calculations which are beyond the scope of this paper.…”