Kinetics of OH- ions in nearly stoichiometric LiNbO3 crystals

Kinetics of the complex OFF stretching vibrational band in a nearly stoichiometric LiNbO3 crystal (Li/Nb approximate to 0.988) has been studied in order to explore the relation between the thermal fixing process of holograms and the hydroxyl ion absorption. By a careful analysis of the spectra we have observed that the intensity of the absorption band components at a given temperature changes with time suggesting that protons migrate among the different sites until they reach a thermodynamic equilibrium. The time dependence of the intensities has been measured at different temperatures between 40-120 degreesC after a heat treatment at 250 degreesC. From the measured data the time constants and the corresponding activation energies (E-av approximate to 1.1 +/- 0.1 eV) were determined and compared with those of the thermal fixing process. The results obtained confirm that the hologram fixing process is governed by proton migration in the crystal.