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

Z.G. Bazhanova^a, A.R. Oganov^b, O. Gianola^c
^aScientific Research Computer Center, Lomonosov Moscow State University, Leninskiye Gory 1, building 4, Moscow, 119991, Russian Federation
^bDepartment of Geosciences and Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York, USA
^cInstitut für Geochemie und Petrologie, Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092, Zürich, Switzerland

The solid inner core of 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 were contradictory. Here, using ab initio simulations, we study the Fe—C and Fe—H systems at inner core pressures (330–364 GPa). Based on the evolutionary structure prediction algorithm USPEX, we have determined the lowest-enthalpy structures of all possible carbides (FeC, Fe₂C, Fe₃C, Fe₄C, FeC₂, FeC₃, FeC₄, Fe₇C₃) and hydrides (Fe₄H, Fe₃H, Fe₂H, FeH, FeH₂, FeH₃, FeH₄) and have found that Fe₂C (space group Pnma) is the most stable iron carbide at pressures of the inner core, while FeH, FeH₃, and FeH₄ are the most stable iron hydrides at these conditions. For Fe₃C, 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 predicted here. We have found that FeH₃ and FeH₄ adopt chemically interesting thermodynamically stable crystal structures, containing trivalent iron in both compounds. 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, yielding the upper bound to the C content in the inner core. 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 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 of the core in the range 43.8–46.5. We conclude that carbon is a better candidate light alloying element than hydrogen.

PACS: 61.50.Ah, 61.50.Ks, 61.50.Nw, 61.66.Fn, 64.30.−t, 91.60.Fe (all)
DOI: 10.3367/UFNe.0182.201205c.0521
URL: https://ufn.ru/en/articles/2012/5/c/
Web of Science

000307559000003
Scopus

2-s2.0-84864990959
ADS NASA

2012PhyU...55..489B
Citation: Bazhanova Z G, Oganov A R, Gianola O "Fe — C and Fe — H systems at pressures of the Earth's inner core" Phys. Usp. 55 489–497 (2012)

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%0 Journal Article
%T Fe - C and Fe - H systems at pressures of the Earth's inner core
%A Z. G. Bazhanova
%A A. R. Oganov
%A O. Gianola
%I Physics-Uspekhi
%D 2012
%J Phys. Usp.
%V 55
%N 5
%P 489-497
%U https://ufn.ru/en/articles/2012/5/c/
%U https://doi.org/10.3367/UFNe.0182.201205c.0521

Received: 13th, December 2011, revised: 22nd, February 2012, accepted: 17th, February 2012

Оригинал: Бажанова З Г, Оганов А Р, Джанола О «Системы Fe — C и Fe — H при давлениях внутреннего ядра Земли» УФН 182 521–530 (2012); DOI: 10.3367/UFNr.0182.201205c.0521