Phase stability and structural properties of Fe2S and its analog Co2P at high pressures and temperatures
Authors: Zurkowski Claire , Lavina Barbara , Chariton Stella , Greenberg Eran , Tkachev Sergey , Prakapenka Vitali , Campbell Andrew 
Published: March 2021

&lt;p&gt;Earth&amp;#8217;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&amp;#8217;s composition and dynamics. In the current study, we examined the high-pressure polymorphism of Fe&lt;sub&gt;2&lt;/sub&gt;S 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 Co&lt;sub&gt;2&lt;/sub&gt;P as a low-pressure analog of Fe&lt;sub&gt;2&lt;/sub&gt;S. Analyses of the powder X-ray diffraction patterns indicate an orthorhombic Fe&lt;sub&gt;2&lt;/sub&gt;S phase coexisting with Fe between 25 and 170 GPa at moderate temperatures. Above&amp;#160; 85 GPa, the orthorhombic Fe&lt;sub&gt;2&lt;/sub&gt;S phase transitions to a hexagonal lattice that is stable on the liquidus to 140 GPa. Using single-crystal diffraction techniques, the orthorhombic structure of Fe&lt;sub&gt;2&lt;/sub&gt;S was solved and refined to the &lt;em&gt;C&lt;/em&gt;23 structure (Co&lt;sub&gt;2&lt;/sub&gt;P type, &lt;em&gt;Pnma&lt;/em&gt;, &lt;em&gt;Z&lt;/em&gt;=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 &lt;em&gt;C&lt;/em&gt;22 Fe&lt;sub&gt;2&lt;/sub&gt;S phase (Fe&lt;sub&gt;2&lt;/sub&gt;P type, &lt;em&gt;P&lt;/em&gt;-62&lt;em&gt;m&lt;/em&gt;, &lt;em&gt;Z&lt;/em&gt;=3), confirming the phase relations inferred in our powder diffraction experiments. The &lt;em&gt;C&lt;/em&gt;23 Fe&lt;sub&gt;2&lt;/sub&gt;S unit-cell parameters fit between 25 and 170 GPa reveal a highly compressible &lt;em&gt;a&lt;/em&gt; axis, where the &lt;em&gt;a&lt;/em&gt; axis is about 3 times more compressible than the &lt;em&gt;b&lt;/em&gt; and &lt;em&gt;c&lt;/em&gt; axes. To 48 GPa,&lt;em&gt; C&lt;/em&gt;23 Co&lt;sub&gt;2&lt;/sub&gt;P shows analogous anisotropic compression behavior to that observed at higher pressures in &lt;em&gt;C&lt;/em&gt;23 Fe&lt;sub&gt;2&lt;/sub&gt;S. Structural analysis of Co&lt;sub&gt;2&lt;/sub&gt;P demonstrates that the anisotropic compression of these C23 phases is attributable to bond angle distortion and bond length compression parallel to the &lt;em&gt;a &lt;/em&gt;direction and that the Co&lt;sub&gt;2&lt;/sub&gt;P-type structure is compressing towards a Co&lt;sub&gt;2&lt;/sub&gt;Si-type structure. These results display the mechanism for anisotropic compression observed in &lt;em&gt;C&lt;/em&gt;23 Fe&lt;sub&gt;2&lt;/sub&gt;S and support previous observations of a &lt;em&gt;C&lt;/em&gt;37-like Fe&lt;sub&gt;2&lt;/sub&gt;S phase above 190 GPa. Through this work, we determined that Fe&lt;sub&gt;2&lt;/sub&gt;S 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 Fe&lt;sub&gt;2&lt;/sub&gt;S and Co&lt;sub&gt;2&lt;/sub&gt;P analogs provides insight into the material properties and dynamics of Earth&amp;#8217;s complex core.&lt;/p&gt;