Energetics Calculations Of Defects Of Crystals And Defect Structures Of Lithium Niobate

This dissertation is concentrated on empirical calculations of ionic crystals in the scale of atoms. It include the choice of empirical models(there are mainly shell model, integral and fractional ionic charge model), the determination of the inter-ionic potential parameters and the model parameters by parameterization method, the structure optimization of perfect crystals, and the core problem of this dissertation?the calculations of the formation energies of point defects in ionic/semi-ionic crystals.Adopting recent experimental data the dissertation studies the physical properties and the point defects of alkaline earth fluorides. The calculated crystal properties in terms of our empirical parameter set are closer to the experimental data than those early results. Then we extend the researches to rutile Ti02. We discuss the error-range of the calculated defect energies besides obtaining more reasonable parameter set. Therefore, adequate preparations are made for studies of low-symmetrical ternary oxide lithium niobate.Lithium niobate, as a technologically important crystalline material for its good physical properties, has high concentration of disorders. Many intrinsic and extrinsic defect models are proposed from different experimental results, but are not often confirmed because they conflict with each other. In order to gain the exact formation energies of point defects a modified relaxed fitting method is employed to get interionic and shell model parameter set. Then the formation configurations and energies of point defects corresponding to different defect models are calculated and the results are: Li Frenkel disorders are the intrinsic defects in stoichiometric LiNbO3 and Li vacancy model is the dominant defect species in congruent LiNbO3. The conclusions are strengthened by using fractional ionic charge model. Applying these two models we also determine the aggregating configuration of the intrinsic defects in congruent LiNbO3 crystals.In the last part of the dissertation the microscopic structures of some kinds of extrinsic defects in lithium niobate are studied. Firstly, an incorporation model of H~ into the LINbO3 crystals is proposed. Besides the judgement from energetics we calculate the vibrational frequency of bydroxy group, which can compare to the 0H absorption spectra. And find the relation between peaks of the hydroxy absorption spectra and the combining configurations of hydroxy group and the intrinsic defects. Secondly a defect structure model in Mg-doped LiNbO3 crystals is raised based on the Li vacancy model. Finally, the proton抯 incorporating position in heavily Mg-doped LiNbO3 is determined: the cluster that make affect the peak of hydroxy absorption spectra is Mg~bII, but not MgHgL1.