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

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.