Revealing the Complex Nature of Bonding in the Binary High-Pressure Compound ${\mathrm{FeO}}_2$

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series $n\mathrm{FeO}m{\mathrm{Fe}}_2{\mathrm{O}}_3$ and the appearance of ${\mathrm{FeO}}_2$. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional $\text{theory}+\text{dynamical}$ mean-field theory calculations, we demonstrate that iron in high-pressure cubic ${\mathrm{FeO}}_2$ and isostructural $\mathrm{Fe}{\mathrm{O}}_2{\mathrm{H}}_{0.5}$ is ferric (${\mathrm{Fe}}^{3+}$), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.

Figure 1

Compressional behavior of $\mathrm{HP}\text{−}\mathrm{Pd}{\mathrm{F}}_2$-type ${\mathrm{FeO}}_2$: (a) lattice volume and $\mathrm{O}─\mathrm{O}$ distance obtained by $\mathrm{DFT}+\mathrm{DMFT}$ at $T=1160\mathrm{K}$ in comparison to XRD data. $\mathrm{DFT}+\mathrm{DMFT}$ predicts a spin-state transition at $\sim 14\mathrm{GPa}$, depicted by a dashed vertical line. (b) Pressure dependence of volumes of the ${\mathrm{FeO}}_6$ octahedra in various compounds according to Ref. [46].

Figure 2

Fe $3d$ ($a_{1g}$, $e_g^{\pi }$, and $e_g^{\sigma }$) and O $2p$ (blue shaded area) spectral functions of $\mathrm{HP}\text{−}\mathrm{Pd}{\mathrm{F}}_2$ ${\mathrm{FeO}}_2$ as obtained by $\mathrm{DFT}+\mathrm{DMFT}$ for different pressures.

Figure 3

(a) Crystal structure and valence electron density plot for $\mathrm{HP}\text{−}\mathrm{Pd}{\mathrm{F}}_2$ ${\mathrm{FeO}}_2$ obtained by $\mathrm{DFT}+\mathrm{DMFT}$ at $\sim 70\mathrm{GPa}$. (b) DFT hybride potential Heyd–Scuseria–Ernzerhof HSE03 results for pyrite-type ${\mathrm{MgO}}_2$ peroxide at ambient conditions. Max stands for 15% of maximum of charge density $\rho (\mathbf{r})$.

Figure 4

Normalized x-ray absorption spectra of Fe ${\mathrm{K}}_{\alpha }$ edge for $\mathrm{Fe}{\mathrm{O}}_2{\mathrm{H}}_{0.5}$ synthesized at 86(2) GPa and 1700(200) K and starting FeOOH (DAC5, see Table S1). Centroid positions are 7114.54(9) and 7114.76(73) eV, correspondingly. Bottom right insets: (a) Microphotograph of $\mathrm{Fe}{\mathrm{O}}_2{\mathrm{H}}_{0.5}$ sample; (b),(c) XAS absorption contrast and absorption jump maps of a sample chamber (palette reflects the relative values of the absorption jump).