| Summary: | 博士 === 國立臺灣師範大學 === 物理學系 === 97 === In order to determine the defect structure of ZnO-doped LiNbO3 single crystals, EXAFS, FTIR, Proton Exchange, Thermal Effect, and Coercive Field experiments were used to target this subject. The calculation of hybrid density functional theory OH─ absorption mode and IFEFFIT EXAFS analysis fitting were also included. From the Extended X-ray Absorption Fine Structure (EXAFS) measurement at room temperature, we find that there is no obvious difference between Zn and Nb core in EXAFS spectra, implying that doped Zn atom is substituted directly on the Li site of LiNbO3 crystal after Zn-doping. An investigation of the OH¯ absorption spectra of Zn-doped LiNbO3 single crystals after proton exchange (PE) is carried out. Before PE treatment, the absorption bands are found centered at approximately 3485 cm─1 below 7.5 mol % concentrations, whereas two distinct bands at 3505 and 3530 cm─1 are clearly observed above 7.5 mol %. After PE treatment, an absorption band at 3505 cm─1 is predominant for all the samples, and this is attributed to the high concentration of H＋ ions substituting Li atoms. For highly Zn-doped samples, the lineshape and intensity of the 3530 cm─1 mode remain the same during PE. From Coercive Field (CF) measurement, large numbers of ZnLi＋atoms of highly Zn-doped samples were moved leading to change OH¯ spectra nearby Nb vacancy structure. For lower doping samples, only fewer NbLi4+ atoms can move, so lower intensities of the OH¯ absorption areas was shown. A theoretical investigation using the hybrid density functional B3LYP method with a simple cluster structure shows that the origins of the 3485 and 3530 cm─1 absorption modes correspond to the Li- and Nb-vacancy models. IFEFFIT EXAFS simulation by way of analyzing the ZnNb scattering amplitudes also shows that the Zn atom does not substitute the Nb site at highly Zn-doped LiNbO3 single crystals. Based on the summary of our experiments, we propose the VNb5─ model for highly doping Zn-doped LiNbO3. This model is in agreement with the calculation of hybrid density functional theory OH¯ absorption mode and IFEFFIT EXAFS analysis fitting. The Nb vacancies should be considered to be an essential factor in influencing the physical properties of Zn-doped LiNbO3 at levels above 7.5 mol % doping concentration.
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