OH stretching vibrations in LiNbO3 (LN) crystals having various Li/Nb ratios and doped with Mg above the photorefractive threshold concentration were investigated in the 20–500 °C temperature range. Beside the 3534 cm−1 above-threshold band another peak at about 3465 cm−1, known from below-threshold stoichiometric LN, was found to appear at a temperature depending on the Mg surplus concentration and grow further with increasing temperature at the expense of the 3534 cm−1 band. The energy difference between the OH− ions was determined to be ΔE=0.25_0.02 eV independently from the composition and Mg concentration. A kinetic model describing the thermal process predicts a linear Mg surplus dependence of the ratio of band areas at a given temperature.
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