Study Of The Effect Of Chemical Reaction On The Crystallization Behaviors Of Lithium Niobate

Lithium niobate has drawn continuous research attention due to its excellent electric and optical properties. The main challenge in this area is how to precisely control morphologies, compositions, and crystal structures, and thus effectively tailor its physical and chemical properties in a controllable way. In this PhD dissertation, some experimental work has been carried out on the synthetic methods including both solution and solid-state reaction routes, to prepare near-stoichiometric lithium niobate and to design one- and three-dimensional lithium niobate micro/nano-structures. The formation mechanisms of various lithium niobate products were also proposed. The main results are summarized herein:One-dimensional rods and three-dimensional hollow spheres of lithium niobate have been fabricated in a versatile amine-assisted hydrothermal system. Various morphologies could be achieved by selecting ligands with different geometrical structures. Additionally, the optical properties of lithium niobate with different morphologies have also been investigated. It has been found that the optical property can be well tuned by adjusting the morphology.Solid-state combustion method has been developed to prepare lithium niobate. In the reaction, urea plays two roles: one is as a combustion reagent, and the other is to coordinate with raw materials, which abruptly reduces the reaction active energy. The operation parameters were comprehensively studied, and the optimal operation parameter has been proposed: reaction temperature about 500-600°C; quantity ratio of urea to raw materials (niobium source and lithium source) about 3:1-4:1; reaction time about 2.5-4 h.A sacrificial template method has been developed in the current work to fabricate niobium oxide and lithium niobate with hollow structure. First, a novel niobium-based precursor H₂(H₂O)Nb₂O₆ with hollow architecture has been designed, and then niobium oxide and lithium niobate hollow spheres were obtained through a dehydration process and a combustion treatment, respectively. The optical analysis results demonstrate that the hollow spheres obtained by this combustion process show high quality, compared with those lithium niobate bulk and lithium niobate hollow spheres fabricated through a solvothermal system. Furthermore, this method can be extended into the fabrication of other niobates with hollow structure due to the high reactivity of precursor. Chemical synthesis methods have been studied on the preparation of near-stoichiometric lithium niobate powders. The effect of heating rate on the composition of products has been studied in detail and the conclusion is that the lithium defect decreases with reducing the heating rate, and under the heating rate of 5-10℃/min, the high quality lithium niobate can be obtained. In addition, the sol-gel method has also been confirmed to be an effective route for fabricating high quality lithium niobate considering that the ligand well coordinates with metal ions, thus effectively protect the lithium lattice.