Investigation of Structural Re-ordering of Hydrogen Bonds in LiNbO₃:Mg Crystals Around the Threshold Concentration of Magnesium

Crystals of LiNbO_3congr and LiNbO₃:Mg (0.19–5.91 mole %) were studied by IR and Raman spectroscopy. It was found that the intensities of the bands corresponding to the stretching vibrations of the ОН groups in the IR spectra of LiNbO₃:Mg crystals change and components of the bands disappear with increase of the Mg content. This was explained by disappearance of the ОН groups close to ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}-{\mathrm{V}}_{\mathrm{Li}}^{-}$ defects as a result of displacement of Nb_Li defects by Mg cations. In the Raman spectra of the LiNbO₃:Mg (5.1 mole %) compared with the congruent crystal the lines corresponding to the vibrations of oxygen atoms in the oxygen octahedra and the stretching bridge vibrations of the oxygen atoms along the polar axis become broader, and new low-intensity lines that may correspond to pseudoscalar vibrations of A₂-type symmetry also appear. The broadening of the lines is due to deformation of the oxygen octahedra caused both by increase of the Mg content in the crystal structure and by change in the localization of the protons. Suppression of the photorefraction effect in the LiNbO₃:Mg crystals with Mg contents above the threshold level can be explained by change in the localization of the protons in the structure and by screening of the space charge field.