A-site Cation Disorder And Size Effects Upon Phase Formation Of Bilayered Perovskite Manganites

The study of doped manganites was started in 1950. The renewed surge of interest in manganites in the 1990s started with the experimental observation of colossal magnetoresistance effects, mostly because it is likely for their great value of being in application or application in the future. On the other hand, as a kind of strongly correlated electron systems, manganites present many intriguing physical properties, and the studies of these physical properties are also of great importance for fundamental scientific research. In this thesis, on the basis of experimental preparation and related properties investigation of bilayered perovskite mangnites, we explore the factors which affect the formation of layered structure, including lattice matching and samples'preparing processes, etc.. And the thesis can be divided into five chapters.Chapter one: At the beginning of this chapter, we reviewed the research progress of perovskite manganites doped with alkaline earth elements, then introduced its physical properties-including crystalline structure, electron structure, phases diagram and electromagnetic behaviors, and related interactions in the system. After that we introduced some elementary properties of layered manganites. It is generally accepted that the double-exchange interaction and the Jahn-Teller distortion are the essential factors and that the phase separation based upon competition between various interactions cannot be neglected, although the opinions about mechanism of manganites are not identical by nowadays. The layered perovskite manganites bring some new contents not only in the fundamental research but also in their applications.Chapter two: At the beginning of this chapter, we briefly present the process of solid state reaction method and some relevant instructions in practice. Then we particularly introduce the Rietveld method for the structural refinement using x-ray diffraction data and the process of using GSAS software and some relevant instructions in practice are also introduced. At last, the measurement processes of electromagnetic properties and some relevant instrument are also presented.Chapter three: The samples with nominal compositions of La_1.4Ca_1.6Mn₂O₇ and La_0.7Ca_0.3MnO₃ were prepared by solid state reaction method and their crystalline structure and relevant properties were compared with each other. It is found that, the sample with nominal composition of La_1.4Ca_1.6Mn₂O₇ is in fact of a multiphase structure composed of La_0.66Ca_0.34MnO₃ and CaO, and the former one dominates the system's properties. Its electromagnetic behaviors support the argument too. We consider that the lattice mismatch of larger La³⁺ ion and smaller Ca²⁺ leads to such results.Chapter four: The sample Ca₃Mn₂O₇ was prepared by the solid state reaction method and its crystalline structure was studied. It is found that Ca₃Mn₂O₇ forms with the Sr₃Ti₂O₇-type tetragonal crystal structure. Then Y_1+xSr_2-xMn₂O₇ (x=0, 0.2, 0.4) series were studied to investigate the relationship between the stability of layered structure of manganites and the matching of cations in A-site. It is shown that the radius difference between A-site cations can't be too large, otherwise the Sr₃Ti₂O₇-type layered structure will loss stable support. At last, the crystalline condition of LaCa₂Mn₂O₇ was studied to investigate the effects of sintering temperature and sintering time on the formation of layered structure. A phase balance process is proposed and thought to exist in generic layered structure phase formation process. And it is thought that the phase balance is dynamic and cannot stop to form a single phase.A summary of this thesis has been given in chapter five.