Optical Anomalies in LiNbO3:Mg Crystals

Abstract

We have observed that LiNbO₃:Mg (0.19–5.91 mol % MgO) crystals experience rearrangements in their defect structure with increasing concentration of Mg and exhibit optical anomalies related with them, which have a threshold character. We have shown that, at Mg concentrations close to the first concentration threshold at ~3.0 mol % MgO, the optical homogeneity of the crystals increases. When the Mg concentration approaches the second concentration threshold, we have revealed a significant shift (by ~50 cm^–1 compared to that in other crystals) of the entire IR absorption spectrum in the range of stretching vibrations of OH groups to the high-frequency range, a sharp decrease in the number of protons, and a jumpwise shift of the absorption edge in the short-wavelength range. We have shown that the shift occurs because all Nb_Li point defects are replaced by Mg cations near the concentration threshold at 5.5 mol % MgO, and, subsequently, Mg cations occupy only positions of Li and Nb cations of an ideal stoichiometric structure, forming Mg_Li and Mg_Nb point defects.