Possible Control of Redox Conditions in the Laser-Heated Diamond Anvil Cell

The redox conditions of Earth have been changing since proto-Earth's accretion from the solar nebula. It has influenced the distribution and partitioning of volatile elements between the atmosphere and the mantle [1,2]. Despite this importance, there have been no methods to control the redox conditions in the laser-heated diamond anvil cell (LHDAC). We have examined feasibility of controlling redox conditions in the LHDAC using mixtures of Ar and H₂ for insulation media at the GSECARS sector of the Advanced Photon Source. In our experiments, ε-FeOOH and Fe₂O₃ starting materials were used for probing changes in the redox conditions. We have also conducted similar experiments with a pure Ar medium to provide a reference point for data that has uncontrolled redox conditions. Our results for the ε-FeOOH starting material in Ar show transformation to Fe₃O₄ and α-Fe₂O₃ with minor Fe₄O₅, while in Ar + H₂ it transformed to Fe₄O₅ with minor α-Fe₂O₃ (hematite) and ι-Fe₂O₃ (Rh₂O₃(II)-type) at 30 GPa and 2000 K. For Fe₂O₃ in Ar + H₂, we found conversion to ε-FeOOH and Fe₅O₆ at 21 GPa and 1600 K. These results demonstrate that H in an Ar medium can promote the conversion of some Fe³⁺ to Fe²⁺. However, the formation of ε-FeOOH in our Fe₂O₃ starting material suggests that H may participate in the chemical reaction of iron oxides.

References:

[1] Righter et al., Elements, 2020.

[2] Stagno et al., Elements, 2020.