High-pressure evolution of ${\text{Fe}}_2{\text{O}}_3$ electronic structure revealed by x-ray absorption

We report the high-pressure measurement of the $\text{Fe}K$ edge in hematite $\left({\text{Fe}}_2{\text{O}}_3\right)$ by x-ray absorption spectroscopy in partial fluorescence yield geometry. The pressure-induced evolution of the electronic structure as ${\text{Fe}}_2{\text{O}}_3$ transforms from a high-spin insulator to a low-spin metal is reflected in the x-ray absorption pre-edge. The crystal-field splitting energy was found to increase monotonically with pressure up to 48 GPa, above which a series of phase transitions occur. Atomic multiplet, cluster diagonalization, and density-functional calculations were performed to simulate the pre-edge absorption spectra, showing good qualitative agreement with the measurements. The mechanism for the pressure-induced electronic phase transitions of ${\text{Fe}}_2{\text{O}}_3$ is discussed and it is shown that ligand hybridization significantly reduces the critical high-spin/low-spin transition pressure.

Figure 1

(Color online) (A) X-ray $K$-edge absorption spectra of ${\text{Fe}}_2{\text{O}}_3$ in partial fluorescence yield geometry at 11 and 64 GPa; Inset: $\text{Fe}K$-edge position at different pressures. The edge is determined by the maximum of the first derivative of the absorption spectra. (B) X-ray $K_{\beta }$ emission spectra of ${\text{Fe}}_2{\text{O}}_3$ at 64 and 0 GPa from Ref. 2, showing the reduction in the spin moment. Red: high-spin state and blue: low-spin state.

Figure 2

(Color online) X-ray $K$-edge pre-edge of ${\text{Fe}}_2{\text{O}}_3$ at 0, 17, 29, 40, 48, 56, and 64 GPa. The bottom three spectra are from SPring-8 using high-resolution monochromator and the top four spectra are from APS using diamond monochromator.

Figure 3

(Color online) (a) Energy of LS state for the single atom multiplet calculation (dotted line) compared with the ${\text{FeO}}_6$ cluster diagonalization (dashed line) relative to the HS state (solid line). (b) HS-LS phase diagram for ${\text{Fe}}_2{\text{O}}_3$. The dotted line shows the probable trajectory of $\left(10Dq,V_{pd\sigma }\right)$ with increasing pressure (see text). [(c)–(e)] $K$-edge pre-edge XAS spectra from the ${\text{FeO}}_6$ cluster calculation at various pressures; $E_A$ is the $\text{Fe}K$-edge absorption energy. (c) At ambient pressure, the spectrum shows distinct $t_{2g}\text{−}{e}_g$ absorption peaks separated by 1.4 eV, indicating a high-spin ground state. (d) At 48 GPa, the peak separation is 1.6 eV, and the ground state still resides in the high-spin sector. (e) At 76 GPa, the spectrum shows broad, multiple peaks, indicating a low-spin ground state. All the spectra were broadened with a 0.3 eV Lorentzian.

Figure 4

(Color online) Ab initio calculations with the FEFF software of the pre-edge region of the $\text{Fe}K$-edge XAS spectra for the high-pressure metallic ${\text{Rh}}_2{\text{O}}_3\left(\text{II}\right)$-type structure of ${\text{Fe}}_2{\text{O}}_3$. $E_F$ is the Fermi level.