X-ray absorption near edge spectroscopy investigations of valency and lattice occupationsite 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 ofdifferentiation and integration methods applied to their Fe K-edge XANES (x-rayabsorption near edge spectroscopy) spectra. The obtained valences are confirmedfurther by absorption measurements in the IR/vis (infrared and visible) spectralrange. It is shown that reduction and thermo-electric oxidization of as-grownFe:LiNbO3, respectively, leads to a noticeable lowering of the number ofFe3+ and to completeoxidization 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 tothe 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 aco-phase formation in the former one. The results obtained can help to tailorsuch crystals for non-linear optical applications as well as for photorefraction.