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.

Phase Stability and Thermal Equation of State of Iron Carbide Fe3C to 245 GPa

“…• The stability of Fe 3 C is evaluated under dynamic compression, and no density discontinuity is observed up to 245 GPa • Thermal equation of state of Fe 3 C is established by combining dynamic and static compression data • The density of Fe 3 C is 5% lower than that of the inner core, and Fe 3 C is unlikely a component of the inner core predictions (Bazhanova et al, 2012;Weerasinghe et al, 2011) revealed that Fe 7 C 3 might decompose into more stable stoichiometries such as Fe 3 C and Fe 2 C at the inner core pressures. There is currently no consensus concerning the stable iron carbide liquidus phase of the Fe-C system at the ICB.…”

Phase relations of Fe3C and Fe7C3 up to 185 GPa and 5200 K: Implication for the stability of iron carbide in the Earth's core

“…Carbon has been commonly considered as a potential major light element as it is the fourth most abundant element in the solar system and its abundance reaches 3.2 wt % in the carbonaceous chondrites-Ivuna (CI). A number of iron carbide phases (e.g., Fe-rich Fe-C alloys, Fe 3 C, Fe 7 C 3 , or Fe 2 C) have been proposed to be the carbon-bearing host phase in the core [e.g., Wood, 1993;Dasgupta et al, 2009;Lord et al, 2009;Nakajima et al, 2009;Tateno et al, 2010;Bazhanova et al, 2012;Fei and Brosh, 2014;Liu et al, 2016a]. Studies of their high-pressure-temperature (P-T) properties including crystal structures and melting curves are thus essential to understanding Earth's accretion and early differentiation as well as the deep-carbon cycle.…”

Geoscience material structures prediction via CALYPSO methodology

“…The formation of iron hydride FeH under pressure has been achieved several decades ago [24] and in recent years polyhydrides FeH 3 [25] (above 86 GPa) and FeH 5 [26] (above 120 GPa) have been reported. Early DFT calculations focused on known simple structure types [27] but CSP has since contributed meaningfully in exploring the binary Fe-H phase diagram [28][29][30][31] , both motivating and confirming the recent experimental efforts.…”

“…Fe-C. Two relatively simple iron carbides are known and have been subjected to detailed experimental and DFT investigations: Fe 3 C, an iron-rich compound important in metallurgy [33] , and Fe 7 C 3 , which forms at relatively low pressures [34,35] . Searching beyond these known compounds, CSP has been used by two groups to find other candidates for high-pressure Fe-C compounds [28,36] . Both reported a new phase, Fe 2 C, to become stable at inner core pressure conditions, which has not been seen in experiments so far.…”

“…Fe 2 C on the other hand has eightfold coordinated carbon, inside quite irregular Fe 8 polyhedra that fill more of space than in the more iron-rich compounds. and drawn to the same scale [28] . Fe (C) atoms are gold (black) spheres.…”