Lithium Niobate <sub> 3 </ Sub> Nanocrystalline And Dopant Lithium Niobate <sub> 3 </ Sub> Nanocrystals And Performance Research

Lithium niobate (LN) crystal is one kind of important multi-purpose crystal, which possesses the fine piezoelectricity, pyroelectricity, electro-optics, double refraction and nonlinear optical (NLO) properties and can be used to manufacture the Q switch to adjust the laser. Because its Curie temperature reaches as high as about 1200°C, it can work under high temperature but not depolarized and the performance is stable. However, the used LN crystals are obtained with the congruent growth. Congruent LN crystal has massive flaw of Li vacancy and trans-position Nb, which disable many applications of LN crystal.Early studies about it show that Li⁺ has the same ion radius and crystal lattice with Nb⁵⁺ in LN crystal, namely, they are in the deformed octahedra, but the intensity of Nb⁵⁺- Li+ electrovalent bonds is larger than that of Li⁺-O^2-, that is why LN crystal tends to the absence of Li⁺, Czochralski method is widely used to grow deformed crystals, Li/Nb mol ratio is nearly 48.6:51.4. Due to the absence of lithium in crystals, there have been some defects in the crystal to keep balance of charge. The defects will affect peformance of LN to some extent, such as Anti-Optical Damage Threshold, Curie-temperature, phase-matching angles, temperature and phtorefractive sensitivity. Recently, many studies about the crystals show that the performances will be improved to increase Li/Nb ratio.Traditional LN crystals growth adopted Solid-Phase Synthesis with high costs, complicated process, lower growth rate, segregation in dopped LN and Li₂O volatilization.LN is widely used as optical medium combined with its excellent electrooptical and NLO characteristics, and the widely used material will be obtained. Wet Chemical Synthesis is different from Solid-Phase Synthesis with some advantages, such as lower process temperature, high uniformity, composition controllable, accurate adulteration. Due to the upper advantagesw, Wet Chemical Synthesis is widely used in the transparent optical ceramics process and raw materials of crystals.Our studies on the raw materials of LN crystal is based on the advantages of Wet Chemical Synthesis and characteristics of LN crystal. We adopt the sol-gel method to synthesize LN nanocrystals and dopped rare earth, transitional-metal ions LN nanocrystals based on referrences. We get some new conclusions through comparing the characteristics of dopped LN nanocrystals with that of dopped LN crystals. We explore the growth of near stoichiometricLN crystals based on the studies of nanocrystals.The contents of the thesis are as follows:1. High purity NbCl₅, CH₃COOLi.2H₂O and citric acid were used as startingmaterials, the transparent gel was obtained by sol-gel method, and the gels were heat-treated at different temperatures, the LN nanocrystals were obtained and characterized with XRD, TG-DTA, FTIR and TEM. The results showed that the pure LN phase was obtained at 600°C, which was much lower than that of convention (>1000°C), and was much lower than Li₂O boiling point (1347℃). The particles were uniform and the average size was about 50-60 nm.2. High purity Nb₂O₅, Li₂CO₃ and citric acid were used as starting materials. The LN nanocrystals were obtained by pechini method, and characterized with XRD. The results showed that the pure LN nanocrystals were obtained when the gels were heat treated at lower temperatures, the size of nanocrystals was 60-70 nm calculated by Scherer formula, which was the same as the results of the TEM. The nanocrystals were uniform and dispersive with no large agglomeration.3. High purity NbCl₅, CH₃COOLi.2H₂O, Er(NO₃)₃.5H₂O and citric acid were used as starting materials, the transparent gel was obtained by sol-gel method, and the gels were heat-treated at different temperatures, the Er³⁺ dopped LN nanocrystals were obtained and characterized with XRD and TEM. The results showed that the LN nanocrystal samples with excellent crystallization and homogeneity were obtained at lower temperatures. The absorption spectrum was tallied with that of the dopped crystalls, and 1053 nm near-infrared laser was observed with the 514nm excitation wavelength, which was the ⁴I_11/2-⁴I_15/2 transition. The results exhibited that there was a 35 nm red shift compared with that of Er³⁺ dopped LN crystals on the fluorescence spectrum, which was due to the quantum size effects and surface effects of nanocrystals.4. High purity NbCl₅, CH₃COOLi.2H₂O, Nd₂O₃ and citric acid were used as starting materials, the transparent gel was obtained by sol-gel method, and the gels were heat-treated at different temperatures, the Nd³⁺ dopped LN nanocrystals were obtained and characterized with XRD,TEM. The results showed that the LN nanocrystal samples with excellent crystallization and homogeneity were obtained at lower temperatures. The obsorption spectrum was tallied with that of the dopped crystals, 1077 nm near-infrared laser was observed with the excitation of 355 nm wavelength, a 10 nm blue shift compared with that of Er³⁺ dopped LN crystals on the fluorescence spectrum. The reason was the same as Er³⁺ dopped LN nanocrystals.5. High purity NbCl₅, CH₃COOLi.2H₂O, CrCl₃.6H₂O and citric acid were used as starting materials, the transparent gel was obtained by sol-gel method, and the gels were heat-treated at different temperatures, the Cr³⁺ dopped LN nanocrystals were obtained and characterized with XRD and TEM. The results presented that the LN nanocrystal samples with excellent crystallization and homogeneity were obtained at lower temperatures. With TG-DTA of the nanocrystals, the absorption spectrum analysis was observed and used Cr³⁺ as optical ion probe to analyze the purity and the approximate stoichiometry of Li/Nb. The obsorption spectrum of the nanocrystal samples was of difference, because the difference of the location Cr³⁺ was in the nanocrystals lattice.