臺灣博碩士論文加值系統

尖晶石結構的材料一直擁有很豐富的物理特性，不同元素構成的尖晶石結構在特性上也相當迥異，而在結構上，目前觀察出的幾種材料中，Fe3O4 為正常尖晶石結構，相反的，NiFe2O4則被認為是反尖晶石結構，本研究中探討的鈷鐵氧體(CoFe2O4)也被認為傾向於反尖晶石結構，然而，這類型的氧化物中，還可能發生不同種類的原子互相交換位置的行為，因此在結構上有多種可能，不能僅憑藉著實驗或是單靠理論模擬計算就妄下結論，我們決定比較實驗數據和理論計算的結果，實驗上採用的是偏振拉曼光譜，由於不同結構的尖晶石結構在拉曼光譜的表現上差異非常大，配合上群論，可以做一個粗略的篩選，而理論計算則能夠提供更確切的數值，兩者搭配起來後，進行一系列的比較，最終我們認為鈷鐵氧體傾向於短程有序的反尖晶石結構。

CoFe2O4(CFO) has a lot of applications due to its large saturation magnetizations and high Neel temperature. The general formula of CFO usually written as (Co2+1-vFe3+v)tetra(Fe3+2-vCo2+v)octO4, where the subscripts tetra and oct correspond to the tetrahedral and octahedral sites respectively. To determine the CFO structure at room temperature, we can analyze the shift and intensity of the peaks from polarized Raman spectra, with the additional aid of first principle calculation along with the selection rules determined by group theory. The Fe/Co ratio in the octahedral and tetrahedral sites of epitaxial CFO films is usually changed due to the variation of growth conditions or the strain applied on films. Such differences may lead to the Raman spectra difficult to be resolved, and there are still few researches about the phonon spectra of the different compositional CFO. We find that the CFO film on the STO substrates is inverse spinel ,however , the nanocomposite may be the partially inverse spinel structures.

目錄
摘要............I
誌謝...........VI
表目錄..X
圖目錄..XII
第一章緒論......1
第二章 文獻回顧..2
2.1尖晶石結構簡介..2
2.2群論與拉曼選擇定則....6
2.2.1群的基本定義與對稱操作....6
2.2.2群表示理論及特徵表....8
2.2.3諾伊曼定理...11
2.2.4拉曼選擇定則.....12
2.3尖晶石結構實驗與理論回顧.....15
2.3.1 尖晶石結構的拉曼光譜......15
2.3.2 尖晶石結構的理論計算......19
2.3.3 尖晶石結構的立方特性......21
第三章 計算方法與實驗方法.......23
3.1 密度泛函理論簡介....23
3.1.1 Hohenberg-Kohn理論........25
3.1.2 Kohn-Sham 方程式..27
3.2交換-關聯能的近似....28
3.3Hellmann-Feynman理論..29
3.4 VASP簡易計算流程圖..30
3.5 詳細計算過程....31
3.6 實驗方法........32
第四章 結果與討論.......33
4.1 各結構最低能量物理性質........33
4.2 計算聲子頻譜與CFO薄膜拉曼光譜之比較..35
4.2.1正常尖晶石結構.....36
4.2.2 反尖晶石結構-Imma.42
4.2.3 反尖晶石結構-P4122......53
4.2.4 反尖晶石結構-P4m2.......67
4.2.5尖晶石結構受應力狀況的計算結果......79
4.2.6理論計算結果與室溫下實驗數據比較....89
4.2.7結論-CFO/STO 薄膜的結膜....92
4.3計算聲子頻譜與奈米複合材料拉曼光譜之比較..93
4.3.1固定長度下的Imma和P4122長度下的比較...94
第五章 結論....96
參考文獻.......97

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