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

Abstract:

Earth’s core is an 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 to 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 multigrain diffraction with ex-situ chemical analysis. We further conducted multigrain diffraction experiments on Co₂P as a moderate pressure analog to the high pressure behavior of Fe₂S. Pnma Co₂P is stable at moderate temperatures to at least 48 GPa. Upon heating at 30 GPa and 1870 K, Pnma Co₂P transitions to P-62m Co₂P. Experiments to 175 GPa and 2600 K reveal analog structural behavior in Fe₂S, and that these are stable sulfides coexisting with Fe. Both Pnma Co₂P and Fe₂S exhibit anisotropic compression where the a axis is ~3x more compressible than the b and c axes. Structural refinements of Co₂P indicate that the anisotropy is attributable to the broadening of Co–P–Co bond angles along the a axis. Through this work we have determined that Fe₂S, rather than Fe₃S, is stable coexisting with Fe to outer core pressures and high temperatures and, using the Co₂P analog, we conducted an in-depth structural examination of these M₂X compounds relevant to Earth’s core.