Accurate Crystal Structural Study on the Materials Science of Perovskite-Type Crystal by Synchrotron Radiation Powder Diffraction

化学式为ABO_3的钙钛矿氧化物的铁电行为受到A和/或B原子替换的强烈影响。许多功能钙钛矿氧化物已被发现利用这种替代效应。然而，对于铁电性的微观起源，为什么晶体相似而化学不同，这类钙钛矿氧化物表现出非常不同的铁电行为的解释并不完整。从20世纪90年代开始，基于第一性原理计算的理论研究在理解钙钛矿氧化物的铁电不稳定性方面取得了很大进展。我们在实验研究方面也取得了进展，通过x射线衍射获得了精确的晶体结构。本项目利用最大熵法(MEM)/Rietveld方法分析高能同步辐射粉末衍射数据，广泛研究了钙钛矿氧化物及相关材料的电子电荷密度分布与相变的关系。在立方相的反铁电氧化物中观察到两个显著的结构特征，即B-O键上的共价键电子密度显著降低，O原子周围的电荷密度分布各向异性。这些特征可以为理解为什么某种钙钛矿氧化物表现出反铁电相变提供线索。

The ferroelectric behavior of perovskite oxides with the chemical formula ABO_3 is strongly influenced by replacing A and/or B atoms with the other atoms. Many functional perovskite oxides have been discovered taking advantage of this replacement effect. However, for the microscopic origin of the ferroelectricity, incomplete interpretation was presented why crystallographically similar, but chemically different, such perovskite oxides exhibit very different ferroelectric behavior. From the 1990s, theoretical studies based on the first-principles calculations have been making great advances to understand the ferroelectric instability of perovskite oxides. We have been also making advances in experimental studies obtaining the accurate crystal structure by X-ray diffraction.In this project, electron charge density distributions of perovskite oxides and related materials have been widely investigated associated with the phase transitions, by analyzing high-energy synchrotron-radiation powder diffraction data by the maximum entropy method(MEM)/Rietveld method. Two distinctive structural features, that is, significant lower covalent bonding electron density on the B-O bond and anisotropic charge density distributions around O atom, are observed in antiferroelectric oxides in the cubic phase. These features can be a clue to understanding why a certain perovskite oxide shows antiferroelectric phase transition.