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PACS 61.72.-y -Defects and impurities in crystals; microstructure PACS 77.84.Ek -Niobates and tantalates PACS 78.30.-j -Infrared and Raman spectra Abstract -OH − absorption bands of both congruent and near-stoichiometric LiNbO3:Hf crystals were investigated. The abrupt blueshift of OH − absorption bands was found between the doping levels of 2 and 2.5 mol% in the congruent crystals, which confirms the exact optical-damage threshold for Hf doping. The absorption difference of OH − bands between the congruent and near-stoichiometric crystals shows that after Li incorporation H + ions no longer prefer the sites of 3500 cm −1 . Moreover, with the increase of Hf doping in the near-stoichiometric crystals, H + ions were found to gradually transfer to the sites of 3489 cm −1 . These results are explained by the breakdown of the association of Hf − Nb and H + ions and the subsequent redistribution of H + ions around Hf 3+Li ions in V − Li -deficient lattice. Additionally, the link between the intrinsic defects and how they influence the optical-damage threshold upon adding Hf was discussed in detail.

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