EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Phase stability and structural properties of Fe2S and its analog Co2P at high pressures and temperatures

Claire Zurkowski1, Barbara Lavina2,3, Stella Chariton2, Eran Greenberg2,4, Sergey Tkachev2, Vitali Prakapenka2, and Andrew Campbell1
Claire Zurkowski et al.
  • 1University of Chicago, Department of the Geophysical Sciences, United States of America (
  • 2University of Chicago, GeoSoilEnviro Center for Advanced Radiation Sources, Chicago, IL 60637, USA
  • 3X-ray Sciences Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA
  • 4Applied Physics Department, Soreq Nuclear Research Center (NRC), Yavne 81800, Israel

Earth’s core is a Fe-rich alloy with a significant contribution from cosmochemically abundant light elements such as sulfur. Understanding the phase stability and structural properties of iron-rich sulfides at core conditions is critical for assessing the core’s composition and dynamics. In the current study, we examined the high-pressure polymorphism of Fe2S coexisting with Fe to outer-core pressures and high temperatures by combining in-situ powder and single-crystal X-ray diffraction techniques. We further conducted single-crystal X-ray diffraction experiments on Co2P as a low-pressure analog of Fe2S. Analyses of the powder X-ray diffraction patterns indicate an orthorhombic Fe2S phase coexisting with Fe between 25 and 170 GPa at moderate temperatures. Above  85 GPa, the orthorhombic Fe2S phase transitions to a hexagonal lattice that is stable on the liquidus to 140 GPa. Using single-crystal diffraction techniques, the orthorhombic structure of Fe2S was solved and refined to the C23 structure (Co2P type, Pnma, Z=4) at 90 GPa and quenched from 2380 K. While upon quenching at 100 GPa from 2650 K, a hexagonal lattice was identified and indexed to a unit cell compatible with a C22 Fe2S phase (Fe2P type, P-62m, Z=3), confirming the phase relations inferred in our powder diffraction experiments. The C23 Fe2S unit-cell parameters fit between 25 and 170 GPa reveal a highly compressible a axis, where the a axis is about 3 times more compressible than the b and c axes. To 48 GPa, C23 Co2P shows analogous anisotropic compression behavior to that observed at higher pressures in C23 Fe2S. Structural analysis of Co2P demonstrates that the anisotropic compression of these C23 phases is attributable to bond angle distortion and bond length compression parallel to the a direction and that the Co2P-type structure is compressing towards a Co2Si-type structure. These results display the mechanism for anisotropic compression observed in C23 Fe2S and support previous observations of a C37-like Fe2S phase above 190 GPa. Through this work, we determined that Fe2S is the relevant Fe-rich sulfide to at least outer core pressures and high temperatures and assessment of the phase transition and compression behavior of the Fe2S and Co2P analogs provides insight into the material properties and dynamics of Earth’s complex core.

How to cite: Zurkowski, C., Lavina, B., Chariton, S., Greenberg, E., Tkachev, S., Prakapenka, V., and Campbell, A.: Phase stability and structural properties of Fe2S and its analog Co2P at high pressures and temperatures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1862,, 2021.


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