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High Poisson's ratio of Earth's inner core explained by carbon alloying


Geochemical, cosmochemical, geophysical, and mineral physics data suggest that iron (or iron–nickel alloy) is the main component of the Earth’s core1,2,3. The inconsistency between the density of pure iron at pressure and temperature conditions of the Earth’s core and seismological observations can be explained by the presence of light elements1,4. However, the low shear wave velocity and high Poisson’s ratio of the Earth’s core remain enigmatic2. Here we experimentally investigate the effect of carbon on the elastic properties of iron at high pressures and temperatures and report a high-pressure orthorhombic phase of iron carbide, Fe7C3. We determined the crystal structure of the material at ambient conditions and investigated its stability and behaviour at pressures up to 205 GPa and temperatures above 3,700 K using single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, and nuclear inelastic scattering. Estimated shear wave and compressional wave velocities show that Fe7C3 exhibits a lower shear wave velocity than pure iron and a Poisson’s ratio similar to that of the Earth’s inner core. We suggest that carbon alloying significantly modifies the properties of iron at extreme conditions to approach the elastic behaviour of rubber. Thus, the presence of carbon may explain the anomalous elastic properties of the Earth’s core.

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Figure 1: Volume–pressure data for o-Fe7C3 with the fitted third-order Birch–Murnaghan equation of state (K300 = 168(4) GPa, K′ = 6.1(1)).
Figure 2: Nuclear resonance measurements of o-Fe7C3 at high pressure.
Figure 3: Variation of Debye sound velocity vD, bulk sound velocity vΦ, shear wave velocity vS and compressional wave velocity vP of Fe7C3 with density.
Figure 4: Extrapolation of Poisson’s ratio to the densities estimated in the Earth’s inner core using Birch’s law.

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Partial financial support was provided through the German Science Foundation (DFG), the German Ministry of Education and Research (BMBF), and the PROCOPE exchange programme. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities through both the normal user programme and a Long Term Project. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1128799) and Department of Energy—Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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M.M. and Y.N. synthesized the starting materials. E.B., K.G., N.D., L.D. and M.H. performed the SCXRD measurements. C.P., L.D., C.M., I.K., A.K., M.M., R.S., V.C., R.R. and A.C. performed the NIS measurements. C.P., L.D. and V.P. performed the high-pressure, high-temperature powder X-ray diffraction measurements. N.M. performed the TEM analysis of the recovered molten sample. C.P., L.D. K.G., I.K., A.K., A.C. and R.R. performed the SMS experiments. C.P., L.D., E.B. and C.M. performed the data analysis. C.P., L.D. and C.M. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to C. Prescher.

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Prescher, C., Dubrovinsky, L., Bykova, E. et al. High Poisson's ratio of Earth's inner core explained by carbon alloying. Nature Geosci 8, 220–223 (2015).

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