X-ray absorption near edge spectroscopy investigations of valency and lattice occupation site of Fe in highly iron-doped lithium niobate crystals

Abstract

The oxidation state of Fe in highly doped lithium niobate crystals (2 wt% Fe2O3) in the as-grown, reduced, and oxidized states is determined by the combination of differentiation and integration methods applied to their Fe K-edge XANES (x-ray absorption near edge spectroscopy) spectra. The obtained valences are confirmed further by absorption measurements in the IR/vis (infrared and visible) spectral range. It is shown that reduction and thermo-electric oxidization of as-grown Fe:LiNbO3, respectively, leads to a noticeable lowering of the number of Fe3+ and to complete oxidization of all Fe2+ to Fe3+, respectively. It is found that Fe in highly iron-doped lithium niobate samples (0.5, 2, and 4 wt% Fe2O3) is incorporated onto the Li site comparing the experimental XANES spectra to the FEFF8 calculations. The difference of the XANES spectrum of the 4 wt% Fe2O3 doped sample as compared to those of the 0.5 and 2 wt% doped samples is explained by a co-phase formation in the former one. The results obtained can help to tailor such crystals for non-linear optical applications as well as for photorefraction.

Site-selective investigation of site symmetry and site occupation of iron in Fe-doped lithium niobate crystals

“…Results from the EXAFS analysis suggest that the iron atoms are dispersed in the LN:Fe 4 wt % crystal. This contradicts the result of Olimov et al, 7 who reported indications for formation of small iron oxide ͑Fe 2 O 3 ͒ clusters in a LN:Fe 4 wt % crystal. The crystal studied in this paper and by Olimov et al originates from the same crystal boule.…”

“…A recent x-ray absorption spectroscopy ͑XAS͒ study reported the presence of Fe 2+ and Fe 3+ atoms in iron-doped LN crystals and their incorporation into the Li site. 7 However, no site-selective structural information was provided, which would allow to differentiate between the Fe 2+ and Fe 3+ sites.…”

Insight into the Effects of Fe Addition on the Local Structure and Electronic Properties of SrTiO3

“…65−68 We used the integration method to determine the average oxidation state value of Fe ions. 42 The pre-edge peak A observed at the Fe K-edge XANES spectra of the STFO samples and displayed in Figure 5a is related to the 1s(Fe) → 3d(Fe) quadrupole transition, while the shoulder B corresponds to pure 1s(Fe) → 4d(Fe) dipole transition. 27 The Fe K-edge XANES spectra exhibit similar features, except for the samples with x = 0.05 and 0.11, because of the lowest iron content.…”

“…The average oxidation state of iron ions was estimated from Fe K-edge XANES spectra and using the integration method. 42 Fe K-edge EXAFS spectra were recorded between 7100 and 7900 eV with energy steps of 2 eV, whereas EXAFS spectra at the Ti K-edge were collected between 4880 and 5800 eV with energy steps of 2 eV. The EXAFS kχ(k) function was weighted with k 3 before carrying out the Fourier transform (FT).…”

Direct Imaging of Valence‐Sensitive X‐Ray Fluorescence Holograms of Fe3O4

“…Generally, the absorption edge of an element shifts toward higher energies as the valence of the atom increases. In the case of iron oxides, this was confirmed by performing X‐ray absorption near edge spectroscopy (XANES) measurements (Figure d) on iron oxide reference samples (FeO [II] and Fe 2 O 3 [III]) and comparing these with the XANES measurements of reference iron oxides in the literature . At an incident X‐ray energy of 7130 eV, all the Fe atoms are excited and this allows the recording of the total Fe K α hologram.…”