【作者】 胡士奇；

【导师】 冯守华；

【作者基本信息】 吉林大学 ， 无机化学， 2010， 博士

【摘要】 本论文主要研究多重价态锰钙钛矿氧化物的电输运性质及其机理。我们用水热方法合成了多重价态锰钙钛矿氧化物的一系列样品,并在不同的外界条件下进行煺火。在煺火的过程中自然使样品中的氧元素溢出或外界的氧进入,当有氧进入时,使La1-x-y CaxKyMnO3中氧的原子数变成3+δ;而当有氧溢出的,使氧的原子数变成3-δ。这一过程以及δ数值的大小会完全改变化合物的性质,我们发现经过这些处理后,样品的导电性确实发生了变化,而X-射线测试显示样品经过处理后结构没有发生变化。我们认为样品的导电性的变化是氧的增加与缺失导致的。为了探明经过不同煺火条件处理后样品性质的变化的原因,我们对样品进行了X-射线光电子能谱分析。在对样品表面元素含量分析中,我们发现La、Ca、K、Mn元素外界条件的影响不大,不会对样品的性质产生巨大的影响;而氧元素变化很大,原有的晶格氧含量变化很大,同时在晶格氧峰位附近出现另一种氧的峰,判断为0价氧的峰,峰位比晶格氧略高。同时,相应的样品的价带谱也随之发生变化,并且eg电子的权重也发生变化。这一现象充分说明,氧在这一个过程中的重要作用及对样品电输运性质的影响。在研究多重价态锰钙钛矿氧化物过程中,我们发现此化合物具有超流特性,这种超流现象只有在超导体进入超导态时才存在。样品通过的电流密度可达5×104 A/cm2,我们对这种现象提出了理论解释:在外加电场和热的作用下,体系首先形成了长寿命的激子,然后这个激子与电子相互作用,最终在这种作用下两个电子相互作用成对,形成电子对,电子对的导电能力极强,从而出现超流现象。更多还原

【Abstract】 Research on the manganites has revealed many novel phenomenons,for example,colossal magnetoresistance(CMR), at the same time,it has lad to some new physical concepts,such as,double-exchang and Jahn-Teller polaron and so on.Early research mainly forcused on searching for insulating ferromagnets with a large magnetization for high-frequency applications, and now we has put light on finding out and explaining large negative magnetoresistance effects which appear near and below the Curie temperature. Based on the abundant physical properties, the manganites can be used for solid electrolytes, catalysts, sensors and novel electronic materials. Their rich electronic phase diagrams reflect the fine balance of interactions which determine the electronic ground state. These compounds represent, in microcosm, the interplay of experiment,theory and application which is at the heart of solid state physics.This kind of compouds is the core in the area of microscopic world, experiment , theory and applications of the solid state physics.Mixed-valence manganese oxides (R1-xAx)MnO3 (R rare-earth cation, A alkali or alkaline earth cation), with a structure similar to that of perovskiteCaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases.Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin flms has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn- Teller effect.Manganese oxides are generally charge-transfer type, but also close to the Mott-Hubbard-type region. Parent-phase LaMnO3 is a charge-transfer-type insulators. When using a small amount of divalent alkaline-earth metal instead of La, the introduction of hole (mainly in the oxygen), the system’s chemical potential change, until the ground state from an insulator into a metal, occurs insulator-metal transition (IM). This band fills the control of IM changes greatly affected by the system bandwidth. When using different sizes of the alkaline earth metals do an alternative, system bandwidth will be subject to modulation (A bit larger ionic radius, bandwidth is also larger). IM changes would give rise to the threshold required for carrier concentration is small. On this basis, we will be mixed with alkali metal K which is bound to introduce some sort of doping disorder, which disorder effects on the system is unpredictable.We have synthesised a series of manganese perovskite oxides by hydrothermal methods and annealed the samples in different conditions, that was at different temperatures and different atmosphere.In detail,the samples were dealt at 500℃, 700℃, 900℃and at the atmosphere of oxygen,vacuum (with or without graphite) and air.In the process, oxygen spilled or ran into the sample. And here, I think the second occurred at the the atmosphere of oxygen and air,as a result, La1-x-y CaxKyMnO3+δappeared. And La1-x-y CaxKyMnO3-δappeared in the first condition. As to the value ofδ, it depended on the properties of the compound, and such condition also was ture of YBCO which resulted in superconductivity. In the text, we found out that the conductivity changed after the annealing. What was more, the crystal structure had no change by XRD. In the process of electrical test, we could say that the conductivity grow good for the samples annealed at the atmosphere of oxygen and air,and the opposite condition occured at the atmosphere of vacuum (with or without graphite) which could be explained as a result ofδ.In order to clarify the reason of the changing nature through different anneal-treated samples, we use the X-ray photoelectron spectroscopy for element valence analysis. With the element content analysis of the sample surface, we found that La, Ca, K, Mn element peak intensity is slightly change and the peak is not remove strongly by the external conditions of sample handling process and the sample nature enormous impact did not change . In the contrast ,the oxygen environment has been different, the original oxygen content has changed dramatically, a shoulder structure at the high BE side of O 2p3/2 main peak has been observed which we conclude that derived from the zero valence oxygen.Meanwhile, the corresponding valence-band spectra of sample also has been changed. This phenomenon fully demonstrates that the oxygen in this process plays an important role in the electronic transport properties of the sample. In further studies, we also found room-temperature ferromagnetic phase in our samples.In the process, we also found that supercurrent about 5×104 A/cm2 appeared for the manganite perovskite oxide which could occurred in the superconducting state of the superconductor.We could explain as follows: at first the system formed the long-life exciton in the presence of the applied electrical field and heat, then as a result of the interactions of the exiton and the electron ,two elecrons formed a pair showing out the supercurrent.更多还原