| This dissertation has three chapters:the first chapter reviews the related backgrounds of the morphotropic phase boundary, magnetoelectric materials and combinatorial materials science. The following two chapters describe our work on the structural and theoretic research of the (PbTiO3)1-x-(CoFe2O4)x (PTO-CFO) composition spread film, including the morphotropic phase boundary in PTO-CFO composition spread film in the second chapter, and mechanism of formation of morphotropic phase boundary and the origin of dielectric anomaly in the third chapter.Chapter one is the introduction of the related background knowledge. At first, the history and mechanism of morphotropic phase boundary are summarized, and the latest experimental and theoretic progresses are reviewed. Secondly, the definition, history and latest research progress of the magnetoelectric materials are introduced. Thirdly, the basic concept, content and history of the combinatorial materials science are introduced, and the latest progresses in parallel synthesis and high-throughput characterization are reviewed. Combined the applications of combinatorial materials science in ferroelectric materials, our previous work on synthesis and characterization for (PbTi03)1-x-(CoFe2O4)x multiferroics materials library is summarized. Finally, the research outline of this dissertation is summarized.In chapter two, the high resolution synchrotron radiation x-ray diffraction structural characterization and analysis on PTO-CFO magnetoelectric composition spread film are described.According to our previous work, a new diffraction peak was observed at x=0.2. To comprehensively investigate the structural evolution of PTO-CFO magnetoelectric composition spread film, high resolution synchrotron radiation x-ray symmetric diffraction patterns and asymmetric diffraction patterns were collected every2%composition step within0.1<x<0.26. To calculate the in-plane lattice constants and obtain the complete lattice information, asymmetric diffraction measurements were carried out. From these results, we found that a strain-induced metastable pseudo-cubic PbTiO3appears at x-0.2, where the metastable pseudo-cubic PbTiO3and existing tetragonal PbTiO3form a tetragonal-pseudocubic morphotropic phase boundary.Additionally,(PbTiO3)0.8-(CoFe2O4)0.2composite film was fabricated using pulsed laser deposition technique. And temperature-dependent high resolution synchrotron radiation x-ray diffraction measurements were carried out to investigate the temperature dependence of phase in the composite film. The results show that the Curie temperature of PbTiO3in composite film signicantly decreases, as compared with that of bulk PbTiO3, which agrees with reports in literatures. The results also show that the diffraction peak position of the strain-induced metastable PbTiO3phase at room temperature is very close to that of cubic PbTiO3at400℃. Combined the symmetric and asymmetric diffraction measurements of metastable PbTiO3phase, we assume that the metastable phase is a pseudo-cubic phase. According to the measurements on PTO-CFO composition spread film and (PbTi03)0.8-(CoFe204)0.2composite film, we composed the composition-temperature phase diagram of composition spread film.In chapter three, the mechanism of origin of morphotropic phase boundary in (PbTi03)1-x-(CoFe2O4)x composition spread film is investigated.At first, enthalpy of PbTiO3of different phases was calculated using first principles calculation through collaborating with Dr. Xia Sun. The results show that enthalpy of the pseudo-cubic PbTiO3crosses that of the out-plane phase around x=0.16, and is about25meV higher than that of the in-plane PTO in morphotropic phase boundary. It indicats that pseudo-cubic PbTiO3has substantial probability in certain composition range at room temperature, considering that kT at room temperature is about26meV.The relative dielectric constant of PTO-CFO composition spread film, as a function of the CoFe2O4content, was calculated using thermodynamic statistical theory with the input from first principles calculation to reveal the relationship between morphotropic phase boundary and dielectric anomaly. The calculation shows that the dielectric anomaly is resulted from the the dynamical balance between different PbTiO3phases under electric field within morphotropic phase boundary. The rebalance of tetragonal and pseudo-cubic PbTiO3under electric field results in a broad peak in relative dielectric constant near x=0.2, coinciding with our observed dielectric anomaly quite well.To observe the rebalance of different PbTiO3phases, the electric field-dependent high resolution synchrotron radiation x-ray diffraction measurements were carried out on (PbTi03)0.8-(CoFe2O4)0.2composite film. The results show almost no intensity change in the in-plane PTO peak. At the same time, out-plane PTO peak increases slightly, while the strain-induced metastable pseudo-cubic PbTiO3decreases at 40kV/mm. It indicates that part of pseudo-cubic PbTiO3transforms into out-plane PTO under E field. These results can be explained by first principles calculation quite well. |
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