Influence of perovskite structure on luminescence and characteristics of ferroics

• Main Authors: Huang, Tung-Ching, 黃同慶
• Other Authors: Hsieh, Wen-Feng
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/14098380671876051829

博士 === 國立交通大學 === 光電工程系所 === 98 === 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.