Off-congruent LiNbO₃ crystals prepared by Li-poor vapor transport equilibration for integrated optics

X- and Z-cut off-congruent LiNbO₃ (LN) crystals for use for integrated optics were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on 1-mm-thick congruent plates at 1100 °C for durations ranging from 65 to 170 h. The Li₂O contents and crystalline phase in these Li-poor VTE-treated crystals were characterized by using Raman scattering, optical absorption, and powder X-ray diffraction spectroscopy. The studies of optical absorption in the ultraviolet region show that the Li-poor VTE treatments result in different extents of red-shifts of the optical absorption edges (OAEs), and the amount of the red-shift increases with the prolonged VTE duration. The mean Li₂O contents in all VTE samples were evaluated from the measured OAEs. The results show that the VTE-induced reduction of the mean Li₂O content can reach as much as ∼1 mol% for both cases of X-cut and Z-cut, and the mean Li₂O content and the VTE duration follows the linear relations: c_Li = (48.62 ± 0.06) - (0.0052 ± 0.0004)t for the X-cut crystals and c_Li = (48.53 ± 0.08) - (0.0063 ± 0.0005)t for the Z-cut crystals. The out-diffusion of Li shows a slight anisotropy. The diffusivity in a Z-cut crystal is slightly larger. The results of visible absorption, Raman scattering, and powder X-ray diffraction show that all the VTE crystals still retain the LiNbO₃ phase, consistent with the result predicted from the LiNbO₃ phase diagram. The critical VTE duration, longer than which the VTE will induce LiNb₃O₈ phase in the crystal, is roughly evaluated to be about 577 and 476 h for an X-cut and a Z-cut plate, respectively.