Electronic Spin transition in FeO$_{2}$: evidence for Fe(II) with peroxide O$_{2}^{2-}$

TL;DR

This study reveals the pressure-induced spin transition in FeO₂, demonstrating that Fe exists as Fe(II) with peroxide O₂²⁻, through combined experimental and computational methods, advancing understanding of its electronic and magnetic properties.

Contribution

It provides the first experimental evidence and theoretical explanation of Fe's spin state transition in FeO₂, clarifying its oxidation state and electronic structure.

Findings

FeO₂ undergoes a pressure-induced spin transition.

Fe in FeO₂ is identified as Fe(II) with peroxide O₂²⁻.

The study combines synchrotron experiments with advanced computational methods.

Abstract

The discovery of FeO$_{2}$ containing more oxygen than hematite (Fe$_{2}$O$_{3}$) that was previously believed to be the most oxygen rich iron compounds, has important implications on the study of the deep lower mantle compositions. Compared to other iron compounds, there are limited reports on FeO$_{2}$ making studies of its physical properties of great interest in fundamental condensed matter physics and geoscience. Even the oxidation state of Fe in FeO$_{2}$ is the subject of debate in theoretical works and there have not been reports from experimental electronic and magnetic properties measurements. Here, we report the pressure-induced spin state transition from synchrotron experiments and our computational results explain the underlying mechanism. Using density functional theory and dynamical mean field theory, we calculated spin states of Fe with volume and Hubbard interaction $U$ change, which clearly demonstrate that Fe in FeO$_{2}$ consists of Fe(II) and peroxide O$_{2}^{2-}$. Our study suggests that localized nature of both Fe 3$d$ orbitals and O$_{2}$ molecular orbitals should be correctly treated for unveiling the structural and electronic properties of FeO$_{2}$.