Optical anomalies in crystals LiNbO$_{3}$ : Mg

We have established that increase in magnesium concentration in LiNbO$_{3}$ : Mg(0.19-5.91 mol% MgO) crystals leads to restructuring in defective structure and optical anomalies connected with the restructuring. The anomalies have threshold character. Optical uniformity of crystals was shown to increase at Mg concentration close to a threshold one $\sim$3.0 mol% MgO. IR-absorption spectrum in the area of stretching vibrations of OH groups shifts to a high-frequency area ($\sim$50 cm$^{-1}$ in comparison with other crystals), when Mg concentration nears the second threshold. At this sharp decrease in protons amount and jump-like shift of an absorption edge in the shortwave region are also observed. The shift was shown to be caused by the fact that at 5.5 mol% MgO all point defects Nb$_{\mathrm{Li}}$ are substitutes by Mg cations and with an increase in Mg concentration, Mg cations occupy Li and Nb sites forming point defects Mg$_{\mathrm{Li}}$ and Mg$_{\mathrm{Nb}}$.