鈣鈦礦結構對螢光及鐵性材料特性的影響

Abstract

我們利用拉曼散射實驗研究物質尺寸和介面應力效應對鐵性材料(包含鈣鈦礦和尖石礦結構)物理特性的影響，同時也探討因取代而造成結構變化對摻雜在ABO3 鈣鈦礦結構中稀土族元素的發光機制的影響。所有的實驗樣品我們利用簡單的化學溶液合成法加以合成。由於取代效應，利用觀察到螢光和拉曼信號消失的現象，我們發現在鈦酸鑭鉛(Pb0.8La0.2TiO3)薄膜中當摻雜的三價鉺離子(Er3+)超過7 mol%時，會對原先微距(short-range)結構中的鈦原子的移位量造成破壞。 這個由於鈦原子的移位量造成的擾動(disorder)會破壞寄主(host materials)鈣鈦礦結構之中心對稱，進而激發稀土元素的螢光輻射和鈣鈦礦的拉曼訊號。在另外一個取代效應的研究中，當以980nm的半導體雷射激發鉺和鐿雙摻雜的鈦酸鉛、鈦酸鋇、和鈦酸鍶粉末，觀察upconversion螢光光譜時，我們發現發紅光的機制由energy-back-transfer (EBT) 程序所主導。EBT的效率不僅依賴鐿離子的濃度大小，也和鉺離子(Er:4S3/2→4I13/2)與鐿離子 (Yb:2F7/2→2F5/2) 的殼層躍遷能階大小是否吻合息息相關。而這個殼層能階大小是否吻合，則取決於摻雜取代造成結構變化，所產生的晶格場(crystal field)變化對離子的殼層能階的影響程度。 不同尺寸(size)鈦酸鋇奈米顆粒的晶格動力學研究中，當粉末顆粒由140 nm縮減到30 nm時，顆粒的四方晶特性(tetragonality)也隨之變弱，同時伴隨著單位晶胞(unit-cell)體積的膨脹，這也是造成光學聲子的縱模和橫模的分裂隨著顆粒尺寸變小而減少的主要機制；另外我們在這部份的研究中，也發現了A1(TO1)和A1(TO2)的耦合強度隨著顆粒尺寸變小而減少，進而造成A1(TO1)原先在拉曼光譜是以下陷(dip)表現的形式變成在奈米尺寸下是以峰型(peak)展示。 最後，在鈦酸鉛-氧化鐵鈷的多重鐵性系統的研究中，發現兩介質介面間的應力造成的磁電效應(magnetoelectric effect)和介質的聲子頻率位移有很強的相關性。受介面影響的聲子能量和多重鐵性系統中的鐵磁物質的鐵電特性都會受到介面間的應力影響；而這個介面間的應力大小，同時受到兩介質晶格不匹配和介質間的化學鍵鍵結程度的影響。在埋有自成長碟型氧化鐵鈷的鈦酸鉛(disk-3 type structure)的樣品，闡明了兩介質間有強的彈性交互作用。而這個較強的作用造成了它的鐵磁特性和晶格特性都有較大的變化，當鐵磁部分有較大的非對稱性磁頑力差和出現不存在飽和的磁化率，伴隨著的是鐵電部分的聲子存在較大的聲子頻率位移。

We investigated on the physical characteristics of ferroics, ABO3-type perovskite and AB2O4-type spinel, which depend on size and strain effects by Raman spectroscopy, and the emission mechanism of rare-earth doped / co-doped ABO3 films on substitution effect. All the samples discussed in this thesis were prepared by chemical solution methods. In one of the cases of substitution effect, we found destruction to a displacement of Ti in the short-range structure by observing the disappearance of emission and Raman signals when the Er3+ concentration exceeds 7 mol% in sol-gel-derived Pb0.8La0.2TiO3 (PLT) polycrystalline films. The disorder due to a displacement of Ti breaks the centrosymmetry to activate emission of rare earth ions such as Er3+ and Raman modes of perovskites. We found that the symmetry breaking diminishes with introducing more Er3+ ions. Furthermore, in another case of substitution effect, we found that quenching of green upconversion (UC) emission accompanied with enhancement of red UC emission under a 980 nm laser pumping was observed dominated by the energy back-transfer (EBT) process in Er3+ and Yb3+ co-doped PbTiO3 (PTO), BaTiO3 (BTO), and SrTiO3 (STO) polycrystalline powders. The efficiency of the EBT process depends not only on Yb3+ concentration but also on level match of the doped Er3+ and Yb3+ ions caused by the crystal fields with different symmetries. Results of emission spectra and X-ray diffraction (XRD) confirm that the centrosymmetric crystal field arising from reducing tetragonality causes level match of transition 4S3/2→4I13/2 of Er3+ and 2F7/2→2F5/2 of Yb3+ that is responsible for enhancing red UC emission. Moreover, in the case of size-dependent lattice dynamics of BTO nanoparticles, we found, upon decreasing the particle size from 140 nm to 30 nm, the tetragonality of BTO nanocrystallites is reduced accompanied with expanding unit-cell volume, which is the dominant mechanism for reducing giant splitting of longitudinal optical (LO) and transverse optical (TO) phonon modes in BTO system. The weakening coupling of two low-frequency modes among three A1(TO) phonons leads to change the lowest one from a spectral dip to a peak; whereas the increasing coupling strength between two high-frequency modes, repels them farther so that the less reducing in spectral separation. Finally, in the part of strain effect, we found the energy of interfacial phonons and ferromagnetic properties depend on stress due to not only the lattice misfit but also the degree of chemical bonding at the interface between CFO and PTO matrices. The disk-3 type structure, the self-assembled CFO disks embedded in PTO matrix, illustrates the strong elastic interactions between these two phases. The largest nonsymmetrical coercivity Hc and the absence of saturation magnetization Ms of CFO matches with the largest Raman shift of A1(TO2) and A1(TO3) modes of PTO found in the disk-3 type as compared with the CFO and PTO multilayered structure and the CFO particles embedded in PTO matrix.