A位离子无序和尺度对双钙钛矿结构锰氧化物成相的影响
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摘要
钙钛矿结构掺杂锰氧化物的研究始于1950年,特别是上世纪90年代其超大磁电阻效应被发现后,此类材料成了科研工作者关注的焦点之一,这是由于它已有的和潜在的应用价值。同时作为一种强关联体系,锰氧化物还表现出十分丰富的物理内容,对于理解强电子关联体系中的诸多物理问题也具有十分重要的基础科学研究的意义。在本论文中,我们对双层钙钛矿结构锰氧化物的实验制备和相关物性进行研究,探索影响层状结构相形成的因素,主要包括晶格匹配和样品的制备工艺过程等。整个论文共分为五章。
第一章首先回顾了掺杂钙钛矿锰氧化物磁电阻材料的研究进展,介绍了此类材料的各种相关性质——晶体结构、电子结构、相图和电磁性质,及相关的物理机制,并对层状锰氧化物的基本性质作了简要介绍。对于锰氧化物的CMR效应,虽然还没有完整统一的理论机制,但一般认为双交换作用和Jahn-Teller极化子是两个最基本的因素,同时各种相互作用竞争造成的相分离也是一个不可忽视的过程。层状锰氧化物无论在应用上还是在基础研究方面都会带来新的内容。
第二章简要介绍了固相法制备样品的流程及在实施过程中的注意事项。接着介绍了X-ray衍射数据精修的Rietveld方法,具体操作时使用GSAS软件的步骤及注意事项。最后对电、磁性质测量的过程及所使用的仪器做了简单的介绍。
第三章采用固相反应法制备了配比为La_(1.4)Ca_(1.6)Mn_2O_7和La_(0.7)Ca_(0.3)MnO_3样品并对照研究了它们的晶体结构及相关物性。研究发现,La_(1.4)Ca_(1.6)Mn_2O_7体系实际上是由La_(0.66)Ca_(0.34)MnO_3和CaO所构成的复相结构,电、磁性质的测量结果也验证了这个事实。我们认为这是由于La~(3+)和Ca~(2+)的半径差别过大而导致的晶格失配的结果。
第四章采用固相反应法制备了Ca_3Mn_2O_7样品并研究了它的晶体结构,结果显示它成的是层状的四方结构,结合Y_(1+x)Sr_(2-x)Mn_2O_7 (x=0, 0.2, 0.4)系列样品我们探讨了层状钙钛矿结构的稳定性与A位占位阳离子匹配程度之间的关系,即A位占位离子半径不能差别过大,否则层状的结构难以得到稳定地支撑。最后通过LaCa_2Mn_2O_7样品的结晶情况,探索了在层状相形成过程中的影响因素特别是烧结温度及烧结时间对相成分的影响。最后我们还指出在一般在层状相形成的过程中存在一个相平衡过程,该过程是动态的并且是无法完全转化为单一的相成分。
第五章对全文进行了总结。
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_(1.4)Ca_(1.6)Mn_2O_7 and La_(0.7)Ca_(0.3)MnO_3 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_(1.4)Ca_(1.6)Mn_2O_7 is in fact of a multiphase structure composed of La_(0.66)Ca_(0.34)MnO_3 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 La~(3+) ion and smaller Ca~(2+) leads to such results.
Chapter four: The sample Ca_3Mn_2O_7 was prepared by the solid state reaction method and its crystalline structure was studied. It is found that Ca_3Mn_2O_7 forms with the Sr_3Ti_2O_7-type tetragonal crystal structure. Then Y_(1+x)Sr_(2-x)Mn_2O_7 (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 Sr_3Ti_2O_7-type layered structure will loss stable support. At last, the crystalline condition of LaCa_2Mn_2O_7 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.
第一章首先回顾了掺杂钙钛矿锰氧化物磁电阻材料的研究进展,介绍了此类材料的各种相关性质——晶体结构、电子结构、相图和电磁性质,及相关的物理机制,并对层状锰氧化物的基本性质作了简要介绍。对于锰氧化物的CMR效应,虽然还没有完整统一的理论机制,但一般认为双交换作用和Jahn-Teller极化子是两个最基本的因素,同时各种相互作用竞争造成的相分离也是一个不可忽视的过程。层状锰氧化物无论在应用上还是在基础研究方面都会带来新的内容。
第二章简要介绍了固相法制备样品的流程及在实施过程中的注意事项。接着介绍了X-ray衍射数据精修的Rietveld方法,具体操作时使用GSAS软件的步骤及注意事项。最后对电、磁性质测量的过程及所使用的仪器做了简单的介绍。
第三章采用固相反应法制备了配比为La_(1.4)Ca_(1.6)Mn_2O_7和La_(0.7)Ca_(0.3)MnO_3样品并对照研究了它们的晶体结构及相关物性。研究发现,La_(1.4)Ca_(1.6)Mn_2O_7体系实际上是由La_(0.66)Ca_(0.34)MnO_3和CaO所构成的复相结构,电、磁性质的测量结果也验证了这个事实。我们认为这是由于La~(3+)和Ca~(2+)的半径差别过大而导致的晶格失配的结果。
第四章采用固相反应法制备了Ca_3Mn_2O_7样品并研究了它的晶体结构,结果显示它成的是层状的四方结构,结合Y_(1+x)Sr_(2-x)Mn_2O_7 (x=0, 0.2, 0.4)系列样品我们探讨了层状钙钛矿结构的稳定性与A位占位阳离子匹配程度之间的关系,即A位占位离子半径不能差别过大,否则层状的结构难以得到稳定地支撑。最后通过LaCa_2Mn_2O_7样品的结晶情况,探索了在层状相形成过程中的影响因素特别是烧结温度及烧结时间对相成分的影响。最后我们还指出在一般在层状相形成的过程中存在一个相平衡过程,该过程是动态的并且是无法完全转化为单一的相成分。
第五章对全文进行了总结。
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_(1.4)Ca_(1.6)Mn_2O_7 and La_(0.7)Ca_(0.3)MnO_3 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_(1.4)Ca_(1.6)Mn_2O_7 is in fact of a multiphase structure composed of La_(0.66)Ca_(0.34)MnO_3 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 La~(3+) ion and smaller Ca~(2+) leads to such results.
Chapter four: The sample Ca_3Mn_2O_7 was prepared by the solid state reaction method and its crystalline structure was studied. It is found that Ca_3Mn_2O_7 forms with the Sr_3Ti_2O_7-type tetragonal crystal structure. Then Y_(1+x)Sr_(2-x)Mn_2O_7 (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 Sr_3Ti_2O_7-type layered structure will loss stable support. At last, the crystalline condition of LaCa_2Mn_2O_7 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.
引文
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