高濃度鋅掺雜於鈮酸鋰晶體之缺陷結構研究
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2009
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為了研究鈮酸鋰晶體缺陷的結構，我們準備了不同鋅摻雜濃度的鈮酸鋰晶體樣品，並運用Extended X-ray Absorption Fine Structure (EXAFS), Fourier Transformation Infrared Ray (FTIR), Proton Exchange (PE), Thermal Effect (TH),以及Coercive Field (CF)的實驗量測進一步對此課題作探討。 從室溫EXAFS的實驗量測中，我們觀察到以Zn及Nb為中心的吸收光譜並沒有明顯的改變，這的確顯示隨著高濃度鋅摻雜的鈮酸鋰晶體中，鋅仍取代鋰原子的位子。 另外從PE FTIR一系列的實驗中，我們觀測到在低濃度鋅摻雜的鈮酸鋰晶體中，PE的貢獻會使3467 cm─1,3485 cm─1以及3505 cm─1的OH¯吸收振動模數量增加，這對應了鋰缺陷的模型。 但是在高濃度鋅摻雜的鈮酸鋰晶體中，PE的貢獻只會使3467 cm─1以及3505 cm─1的OH¯吸收振動模增加，但對於3530 cm─1的OH¯吸收振動模則不產生影響，這對應了不同缺陷模型存在的結果。 而TH的實驗中我們了解到高濃度鋅摻雜的鈮酸鋰晶體中OH¯吸收振動模的數量小於零摻雜的結果，這說明了高濃度鋅摻雜的鈮酸鋰晶體結構較為緊密，此現象跟X-ray Diffraction在高濃度鋅摻雜的結果吻合。 另外在CF的實驗結果中，我們解釋了高濃度鋅摻雜的鈮酸鋰晶體在CF作用，並搭配鈮缺陷的模型配合下，ZnLi+原子的移動情況。 在Gaussian 03的理論擬合結果中，我們計算出3485 cm─1主要對應了鋰缺陷附近的OH¯吸收振動模，而3530 cm─1的OH¯吸收振動模位於鈮缺陷附近。 最後综合所有實驗結果，我們提出高濃度鋅摻雜的鈮酸鋰晶體中存在鈮缺陷的模型，而此鋅摻雜的濃度需高於7.5 mol %以上。
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.
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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鈮酸鋰晶體, 缺陷結構, OH¯吸收振動模, 高濃度鋅摻雜, vacancy structure, OH─ absorption mode, IFEFFIT EXAFS analysis fitting, hybrid density functional theory