Preparation Of LiNbO₃ With Different Composition Using Vapour Transport Equilibration Technique

Many important physical properties of lithium niobate crystal strongly depend on the composition and lattice defects inside the crystal. Vapor transport equilibration (VTE) is a practical and effective method to alter the Li-composition in a congruent LiNbO₃ crystal. Up to date, a number of papers involving the preparation of Li-rich lithium niobate using vapor transport equilibration technique have been reported. However, few papers could be found in the earlier literatures that involve using the same technique to prepare a Li-deficient lithium niobate, which is also very promising in integrated optical field. Present dissertation reports the research work in this aspect for the first time, to the best of our knowledge. Some preliminary experimental results are reached. These experimental results can offer considerable guidance to the further work in the future.The main work of the dissertation can be summarized as following four aspects:1. Li₂CO₃ and Nb₂O₅ powders were used as the starting chemicals. At first, the two powders were homogeneously mixed according to the molar ratio of 32mol%:68mol%. The mixture was then pressed and molded into a crucible of 6 cm in inner diameter and 4 cm in height. The calcinations at 1000 oC for 10 h and at 1100 oC for 1 h allowed us to prepare a Li-deficient two-phase (LiNbO₃ + LiNb₃O₈) crucible.2. A number of 1-mm-thick X-cut and Z-cut congruent LiNbO₃ plates were VTE-treated by using the prepared Li-poor two-phase crucible. The VTE treatment was carried out at 1100 oC for different durations ranging from 63 to 168 h. Consequently, a number of X-cut and Z-cut Li-poor plates were prepared.3. Raman scattering technique and optical absorption spectroscopy were then used to characterize Li-compositions in these prepared samples. Raman scattering experimental results showed that the 153 cm^-1 E (TO) phonon linewidths (full width at half maximum, FWHM) of all the samples prepared became broadening as compared with those of the corresponding congruent crystal. The longer the VTE duration is, the more obvious the broadening is. This experimental result qualitatively shows that the VTE procedure has induced the reduction of Li compositions inside these plates. Ultraviolet absorption results show that the optical absorption edges (at the absorption coefficient of 20 cm^-1) of these VTE-treated samples exhibit different extent of red-shifts relative to that of the corresponding congruent crystal. The longer the VTE duration is, the larger the red-shift is. The largest red-shift can be as much as 5 nm. A previously reported empirical formula, which describes the relationship of the Li-composition inside the LiNbO₃ crystal and the photon energy corresponding to the absorption, was used to estimate Li composition in all samples. For the X-cut samples, the Li2O content can decline from 48.35 mol% to 47.74 mol% (The value of the congruent material is 48.6 mol %.) as the VTE duration is increased from 63 to 168 h. For those Z-cut samples, the Li₂O content can degrade from 48.07 mol% to 47.50 mol% as the VTE duration is increased from 88 to 168 h. The uncertainty for the measured UV absorption edge is estimated to be 0.3 nm, which yields a Li2O content resolution of 0.04 mol%.4. The Visible optical absorption data show that the background absorption in the sample with the VTE duration of 168 h is considered as unchanged within the absorption coefficient error of±0.2 cm^-1. It can be thus preliminarily anticipated that after the 168 h VTE treatment the transparency of the crystal is retained still and no other non-LiNbO₃ crystalline phase was induced in the crystal. This preliminary speculation was further verified by the subsequent Raman scattering and X-ray powder diffraction results.