镧基钙钛矿型复合氧化物的体相氧缺陷与结构稳定性研究

发布时间：2018-03-22 17:11

本文选题：钙钛矿氧化物　切入点：第一性原理　出处：《华东理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：镧基钙钛矿型复合氧化物是一种优良的氧化还原反应催化剂。具有化学计量比的完美钙钛矿氧化物体相结构中氧摩尔含量为3/5,容易被还原失去晶格氧而形成氧空穴。氧空穴的存在会影响钙钛矿氧化物的物理和化学性质,并且伴随的结构稳定性问题会限制其在实际中的应用。本文采用基于密度泛函理论的第一性原理计算,对9种钙钛矿氧化物LaMO_3(M=Sc-Cu)的结构稳定性随体相氧离子缺位数的变化进行了研究。采用Bader电荷分析氧空穴形成前后的钙钛矿体相电子结构可知,氧移除后电荷的重新分布过程中最邻近氧空穴的两个过渡金属离子的电荷变化最大,并且氧空穴形成能的大小与氧离子留下的电子如何占据过渡金属离子d轨道有关。进一步地,通过将氧空穴形成能分解为弛豫能和键能,发现氧空穴形成能受到键能大小的直接影响。此外,论文研究了体相中氧离子缺位数δ为0.125-1.25的条件下,LaMO_(3-δ)(M=Sc-Cu)晶体结构的稳定性。结果表明,一方面,4种包含过渡金属元素的LaMO_3(M=Sc、Ti、V和Cr)由于具有很高的氧空穴形成能,结构几乎不能被还原;另一方面,结合不同氧离子缺位数条件下的氧空穴形成能变化曲线可以看出:当δ0.5时,4种LaMO_(3-δ)(M =Mn、Fe、Co和Ni)氧化物中氧空穴形成能陡然升高,并且大于4.80 eV,意味着在此条件下它们不能被还原而继续提供氧离子。对于LaCuO_(3-δ)而言,只有当δ增大并超过1.0时,其氧空穴形成能才接近4.8 eV,因此可以提供更多氧离子。若选择4.8 eV作为判断氧离子能否被抽离的氧空穴形成能临界值,并以氧空穴形成能曲线突升前一点定义最大可能氧离子缺位数,则后5种钙钛矿可至多被还原为:LaMnO_2.5、LaFeO_2.5、LaCoO_2.5、LaNiO_2.5和LaCuO_2。LaMO_3氧化物移除氧离子后保持结构稳定性的一个必要条件是过渡金属元素存在不同的氧化态。当LaMO_3被还原为LaMO_2.5,过渡金属离子获得1个电子,过渡金属离子的氧化态从+3降为+2;而当LaMO_3被还原为LaMO_2,过渡金属离子获得2个电子后氧化态被降低为+1。
[Abstract]:Lanthanum based perovskite complex oxide is an excellent catalyst for redox reaction. Perfect perovskite oxide with stoichiometric ratio has an oxygen molar content of 3 / 5, which is easy to be reduced to lose lattice oxygen and form oxygen empty. The presence of oxygen holes affects the physical and chemical properties of perovskite oxides, And the associated structural stability problem will limit its application in practice. In this paper, the first principle calculation based on density functional theory is used. The structural stability of 9 perovskite oxides LaMoS _ 3M _ (Sc-Cu) was studied. The electronic structure of perovskite before and after the formation of oxygen holes was analyzed by Bader charge analysis. The charge changes of the two transition metal ions closest to the oxygen hole in the process of charge redistribution after oxygen removal are the largest. Moreover, the size of the oxygen hole formation energy is related to how the electrons left by the oxygen ion occupy the transition metal ion d orbital. Further, by decomposing the oxygen hole formation energy into relaxation energy and bond energy, It is found that the formation energy of oxygen hole is directly affected by the bond energy. In addition, the stability of the crystal structure of LaMO-3- 未 Mn-Sc-Cu is studied under the condition that the oxygen ion vacancy number 未 is 0.125-1.25. On the one hand, the structure of four kinds of LaMO3MU / TI-V and Cr containing transition metal elements can hardly be reduced due to their high oxygen cavitation formation energy; on the other hand, the structure can hardly be reduced because of their high oxygen hole formation energy, on the other hand, In combination with the oxygen hole formation energy change curves under different oxygen ion vacancy numbers, it can be seen that the oxygen hole formation energy in four kinds of LaMO-T _ 3- 未 ~ (-) M ~ (2 +) O _ (2) O _ (2) O _ (3) O _ (1) O _ (3) O _ (3) O _ (3) O _ (3) O _ (3) O _. And more than 4.80 EV, which means that they can't be reduced under this condition and continue to supply oxygen ions. For LaCuO _ s _ 3- 未), only when 未 increases and exceeds 1.0, The oxygen hole formation energy is close to 4.8 EV, so more oxygen ions can be provided. If 4.8 EV is chosen as the critical value of oxygen hole formation energy to determine whether oxygen ion can be withdrawn, The maximum number of possible oxygen ion vacancies was defined by the point before the oxygen hole formation energy curve suddenly rose. Then the latter five perovskites can be reduced to at most: LaMnO _ s _ _ _. The oxidation state of transition metal ions was reduced from 3 to 2, and when LaMO_3 was reduced to LaMO2, the oxidation state of transition metal ions was reduced to 1.
【学位授予单位】：华东理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O643.36

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