Electronic spin transition in ${\mathrm{FeO}}_2$: Evidence for Fe(II) with peroxide ${\mathrm{O}}_2^{2-}$

The discovery of ${\mathrm{FeO}}_2$ containing more oxygen than hematite $\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$, which was previously believed to be the most oxygen rich iron compound, has important implications for the study of deep lower mantle compositions. Compared to other iron compounds, there are limited reports on ${\mathrm{FeO}}_2$, making studies of its physical properties of great interest in fundamental condensed matter physics and geoscience. Even the oxidation state of Fe in ${\mathrm{FeO}}_2$ 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 ${\mathrm{FeO}}_2$ consists of Fe(II) and peroxide ${\mathrm{O}}_2^{2-}$. Our paper suggests that the localized nature of both Fe $3d$ orbitals and ${\mathrm{O}}_2$ molecular orbitals should be correctly treated for unveiling the structural and electronic properties of ${\mathrm{FeO}}_2$.

Figure 1

(a) X-ray emission spectra for ${\mathrm{FeO}}_2$ as a function of pressure. The satellite peak at around 7040 eV is characteristic of the high-spin state. (b) Pressure-dependent ${\mathrm{FeO}}_2$ volume. The blue dashed line is calculated from nonmagnetic calculation with $\mathrm{DFT}+U$ method $\left(U=5\mathrm{eV}\right)$ for the nonmagnetic high-pressure regime. Its extension to the high spin state low-pressure regime is to guide the eye, in order to demonstrate the volume deviation.

Figure 2

(a) Calculated volume $\left(V\right)$ and Coulomb interaction $\left(U\right)$ phase diagram of ${\mathrm{FeO}}_2$. Colors present the size of the magnetic moment. Open and closed circles indicate the insulating and metallic phases, respectively. The dashed region indicates the experimentally observed transition point. (b, c) Calculated (b) Fe-O bond length and (c) ${\mathrm{O}}_2$ dimer bond length depending on the $U$ value. The arrows in (a) and (c) indicate that the magnetic moment increases as the ${\mathrm{O}}_2$ dimer bond length increases. There are abrupt changes both in Fe-O and O-O bond length at $U=5\mathrm{eV}$. (d) Relative energy depending on the ${\mathrm{O}}_2$ dimer bond length at volume of $24.9{\AA }^3/\mathrm{f}.\mathrm{u}.$ with $U=5\mathrm{eV}$. There are two local minima depending on the ${\mathrm{O}}_2$ dimer bond length.

Figure 3

(a) Calculated $V\text{−}U$ phase diagram of ${\mathrm{FeO}}_2$ using a fixed ${\mathrm{O}}_2$ dimer bond length of 1.8 Å. Open diamonds indicate the Mott insulating state. The orange dash-dotted line indicates the boundary for Fe(II) with peroxide state. The LS region is divided into (b) and (c) regions by the orange dash-dotted line. The electronic structure evolves from (b) LS metallic state to (c) LS band insulator, and finally to (d) HS Mott insulator along the white guide line in (a).

Figure 4

Density of states calculated from $\text{DFT}+\text{DMFT}$ at volume (a) $22.8{\AA }^3/\mathrm{f}.\mathrm{u}.$ (band insulator, $S=0)$ and (b) $23.9{\AA }^3/\mathrm{f}.\mathrm{u}.$ (Mott insulator, $S=2)$. Inset: The calculated temperature dependence of the local spin susceptibility at each volume.