The infrared spectra of congruent LiNbO3:Fe:M (M= Mg2+, Zn2+, In3+, [Li]/[Nb]=0.946) crystals with 0.03wt% Fe2O3 were measured by a FT-IR spectrometer from 3400cm-1 to 3580cm-1. The spectra show that OH- absorption band of double-doped LiNbO3:Fe:M crystal locates at about 3482 cm-1 as that of single-doped LiNbO3:Fe crystals, and it is not shifted to ultraviolet band until the concentrations of M exceeds its concentration threshold. The threshold of damage-resistant ions M is 6mol% Zn2+, 4mol% Mg2+ and 3mol% In3+, respectively. The OH- absorption band of M-doped (>concentration threshold) LiNbO3:Fe crystals shifted to 3535, 3525 and 3508 cm-1, which results from the OH- stretching vibration in [MNb]a--OH--[MLi]b+ complexes. The influence of valence on the concentrate threshold is also discussed.
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