Local structure of magnetite and maghemite and chemical shift in Fe K-edge XANES

摘要

Local structures around Fe and chemical shifts in X-ray absorption fine structure (XAFS) spectra were investigated for synthetic Fe_(1-)_(d )O, Fe_(3)O_(4 )(magnetite), γ-Fe_(2)O_(3)(maghemite), and SrFeO_(3-)_(e ), as well as natural Fe_(3)O_(4) and α-Fe_(2)O_(3) (hematite) specimens. XAFS spectra near the Fe K-edge were measured at BL-9C and BL-12C of the Photon Factory, KEK, Japan. Similar measurements for Fe-N pairs in Fe nitrides (FeN, Fe_(2)N, Fe_(4)N) were obtained for comparison. The X-ray absorption near edge structure (XANES) spectra for various iron compounds, in particular Fe_(3)O_(4) and γ-Fe_(2)O_(3), and Fe_(x )N specimens, showed clear chemical shifts of half-maximum positions with the changing oxidation states of the Fe ions in their structures. The energy at half maximum position should be used for the quantitative discussion of the oxidation and valence states of the Fe ion in Fe compounds rather than the threshold energy E _(0) found using differentiating XAFS spectra. The δ values in Fe_(3-)_(δ )O_(4) (δ =0.333 for end-member γ-Fe_(2)O_(3)) for magnetite and maghemite were estimated by the extent of the chemical shifts. Decreases in the average Fe-O distances and extended X-ray absorption fine structure (EXAFS) Debye-Waller factor σ ~(2) values for spinel-type Fe_(3-)_(δ )O_(4) solid-solutions were consistent with the estimated δ values. The error in proving the Fe~(3+)/Fe~(total) ratio in the system by the XAFS method would appear to be less than ±0.10, and higher reliability than this value would be acquired in the comparison of relative values at half-maximum positions.