Experimental versus $\mathit{ab} \mathit{initio}$ x-ray absorption of iron-doped zirconia: Trends in O $K$-edge spectra as a function of iron doping

We present an experimental study of x-ray absorption near edge structure (XANES) at $L_{2,3},M_{2,3}$, and $K$ edges of, respectively, Fe, Zr, and O in iron-doped zirconia $({\mathrm{ZrO}}_2:\mathrm{Fe})$ for different Fe dopant concentrations $x$ (from $x\sim 6\%$ to $x\sim 25\%$ at.) and make the comparison with ab initio simulations at the O $K$-edge. The x-ray magnetic circular dichroism (XMCD) measurements show no evidence of ferromagnetic (FM) order for all the analyzed samples in agreement with our ab initio simulations, which show an antiferromagnetic (AFM) order. We found that substituting Zr with Fe atoms leads to a radical change in the O $K$-edge XANES spectrum, especially in the pre-edge region where a pre-edge peak appears. This pre-edge peak is ascribed to dipole transitions from O $1s$ to O $2p$ states that are hybridized with the unoccupied Fe $3d$ states. Both theoretical and experimental results reveal that the intensity of the pre-edge peak increases with Fe concentration, suggesting the increase of unoccupied Fe $3d$ states. The increase of Fe concentration increases oxygen vacancies as required for charge neutrality and consequently improves AFM ordering. According to our first-principles calculations, the effect of one Fe atom is mostly localized in the first oxygen shell and vanishes as one moves far from it. Thus the increase of the O $K$-pre-edge peak with increasing Fe concentration is due to the increase of percentage of oxygen atoms that are near neighbors to Fe atoms.

Figure 1

Room temperature XAS spectra recorded in TEY mode for the different Fe-doped samples: (a) Zr $M_{2,3}$, (b) O $K$, and (c) Fe $L_{2,3}$ edges.

Figure 2

Relaxed structures of ${\mathrm{ZrO}}_2$:Fe for different Fe atomic concentrations namely $6.25\%,12.5\%$, and $25\%$, respectively, from left to right. Zr, O, and Fe atoms are, respectively, in gray, green, and red colors.

Figure 3

(a) Calculated O $K$-edge XANES spectra in iron-doped zirconia $({\mathrm{ZrO}}_2$:Fe) at $x=6.25\%$ and in pure zirconia. Note the presence of pre-edge peak and energy shift for the case of ${\mathrm{ZrO}}_2$:Fe. (b) Spin-polarization dependence of the O $K$-edge XANES spectrum of one oxygen atom located in the first shell around one spin-down polarized Fe atom. (c) Löwdin DOS projected, respectively, on the $2p$ orbital of the absorbing oxygen atom and $3d$ orbital of the nearest Fe. The hatched areas in (c) show the PDOS peaks responsible for the pre-edge peak marked as hatched area in (b).

Figure 4

(a) Evolution of O $K$-edge XANES spectrum with respect to iron concentration. The absorption peaks ${\mathrm{B}}_2$ are set to 531.18 eV in order to make a direct comparison with the experimental results. (b) shows that the pre-edge peak originates mainly from the first oxygen shell around the iron atom and a small contribution coming from the second oxygen shell. (c) The mean contributions of the two first oxygen shells in the O $K$-edge spectra.