Abstract
An optical absorption and electron paramagnetic resonance (EPR) study has been performed on a series of chemically reduced congruent lithium niobate (LiNbO 3) samples. Chemical reduction enhances the high-temperature electrical conductivity and darkens an otherwise optically clear crystal. The resulting optical spectra show a sharp threshold absorption above 3.8 eV and the growth of a broad-band absorption featuring a peak at 2.4 eV. The strength of the broad-band absorption is directly correlated with the carriers responsible for the enhanced high-temperature electrical conductivity. Optical illumination (bleaching) at 87 K shifts the optical absorption peak from 2.4 to 1.6 eV and produces an enhanced Nb 4+ EPR signal. A quantitative analysis shows that the 1.6 eV optical absorption and the Nb EPR signal are both consistent with single electrons trapped on Nb atoms. The experimental results are discussed in terms of oxygen vacancy and bipolaron models.
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