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

摘要

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 Kedge 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 thermoelectric 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.