OH^- absorption in off-congruent LiNbO₃ crystals prepared by Li-poor vapor transport equilibration

An X-cut and a Z-cut Li-deficient, non-congruent LiNbO₃ crystal were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on one mm thick congruent plates at 1100 °C for 170 h. From the measured optical absorption edges (OAE), the Li₂O content is evaluated to be 47.74 ± 0.13 mol% in the X-cut plate and 47.61 ± 0.13 mol% in the Z-cut VTE crystal. The composition homogeneity is verified by measuring the OAE as a function of the crystal thickness. The visible optical absorption and powder X-ray diffraction studies allow to conclude that both VTE crystals retain still the LiNbO₃ phase. OH^- absorption characteristics of the two VTE crystals were investigated in comparison with those of the as-grown crystals. The results show that the Li-poor VTE treatment only slightly modifies the area, spectral width, spectral shape and background absorption of the OH^- band. Comparison shows that the Li-poor VTE effects on the OH^- absorption have large differences from the Li-rich VTE effects in bandwidth and areas of three component peaks at 3466, 3481 and 3488 cm^-1. These spectral differences are explained on the basis of the H⁺ site occupation and crystal defect models taking into account the VTE effect on crystal defect concentration. The Li-poor VTE effects are attributed to the VTE inducing the redistribution of OH^- ions in different sites.