Phase Relations in the Fe-Si-H Ternary up to 125 GPa and 3700K: Implications for the Structure and Chemistry of Planetary Cores&#160;
Authors: Fu Suyu , Chariton Stella , Prakapenka Vitali , Chizmeshya Andrew , Shim Sang-Heon 
Published: March 2022

&lt;p&gt;Light elements play a key role in the chemical and physical processes of planetary Fe-rich metallic cores [1].&amp;#160; H and Si are believed important candidates in planetary cores and previous estimates indicate as much as 0.6 wt% H and 13 wt% Si in the Earth&amp;#8217;s core [2, 3]. However, existing studies are on Fe-H or Fe-Si binary systems and knowledge on Fe-Si-H ternary at high pressure and temperature is still limited [4, 5]. We conducted a series of experiments to understand the impact of hydrogen on Fe-Si alloy system. Fe-Si alloys with three compositions, Fe-9Si (9 wt% Si), Fe-16Si (16 wt% Si), and FeSi (33.3 wt% Si), reacted with H separately up to 125 GPa and 3700 K in diamond-anvil cells by combining pulsed laser heating with high-energy synchrotron X-ray diffraction. Results show little H solubility in B20 and B2 phases of FeSi (0.3 wt% and &lt;0.1 wt% H, respectively) up to 62 GPa, which is significantly smaller than H solubility in Fe metal (1.8 wt% H) [6]. The low H solubility in these phases is likely because of their highly distorted interstitial sites which are not favorable for H incorporation. We found that the low-Si alloys (Fe-9Si and Fe-16Si) convert into FeH&lt;sub&gt;x&lt;/sub&gt; (fcc or dhcp), FeSi (B20 or B2), and Fe-Si-H ternary phases up to 125 GPa and 3700 K. Particularly, a Fe&lt;sub&gt;5&lt;/sub&gt;Si&lt;sub&gt;3&lt;/sub&gt;H&lt;sub&gt;x&lt;/sub&gt; phase is stable below 43 GPa and the cubic FeH&lt;sub&gt;3&lt;/sub&gt; can appear after reactions above 100 GPa. These results indicate that H alters the behavior of the Fe-Si system severely. Considering the various sizes and masses of planets in the solar and exoplanetary systems, the planetary cores can have a wide range of Si contents. If Fe-droplets in early magma ocean contain much Si, Si could limit the amount of H incorporated in the core. On the other hand, for cores with low Si, crystallization at the solid-liquid core boundary may result in formation of separate H-rich and Si-rich crystals in the solid core, potentially inducing heterogeneities in the region [7].&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;1. Shahar, A., et al., &lt;em&gt;What makes a planet habitable?&lt;/em&gt; Science, 2019. &lt;strong&gt;364&lt;/strong&gt;(6439): p. 434-435.&lt;/p&gt;&lt;p&gt;2. Tagawa, S., et al., &lt;em&gt;Experimental evidence for hydrogen incorporation into Earth&amp;#8217;s core.&lt;/em&gt; Nature Communications, 2021. &lt;strong&gt;12&lt;/strong&gt;(1): p. 2588.&lt;/p&gt;&lt;p&gt;3. Hirose, K., B. Wood, and L. Vo&amp;#269;adlo, &lt;em&gt;Light elements in the Earth&amp;#8217;s core.&lt;/em&gt; Nature Reviews Earth &amp; Environment, 2021. &lt;strong&gt;2&lt;/strong&gt;(9): p. 645-658.&lt;/p&gt;&lt;p&gt;4. Terasaki, H., et al., &lt;em&gt;Hydrogenation of FeSi under high pressure.&lt;/em&gt; American Mineralogist, 2011. &lt;strong&gt;96&lt;/strong&gt;(1): p. 93-99.&lt;/p&gt;&lt;p&gt;5. Tagawa, S., et al., &lt;em&gt;Compression of Fe&amp;#8211;Si&amp;#8211;H alloys to core pressures.&lt;/em&gt; Geophysical Research Letters, 2016. &lt;strong&gt;43&lt;/strong&gt;(8): p. 3686-3692.&lt;/p&gt;&lt;p&gt;6. P&amp;#233;pin, C.M., et al., &lt;em&gt;New iron hydrides under high pressure.&lt;/em&gt; Physical review letters, 2014. &lt;strong&gt;113&lt;/strong&gt;(26): p. 265504.&lt;/p&gt;&lt;p&gt;7. Deuss, A., &lt;em&gt;Heterogeneity and anisotropy of Earth's inner core.&lt;/em&gt; Annual Review of Earth Planetary Sciences, 2014. &lt;strong&gt;42&lt;/strong&gt;: p. 103-126.&lt;/p&gt;