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其他原子取代A和/或B原子的强烈影响。利用这种替代效应，人们已经发现了许多功能性的钙钛矿氧化物。然而，对于铁电性的微观起源，人们给出了不完全的解释，为什么这些钙钛矿氧化物在结晶学上相似，但在化学上不同，表现出非常不同的铁电行为。自20世纪90年代以来，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。通过X射线衍射法获得准确的晶体结构的实验研究也取得了进展。在本项目中，我们通过最大熵法(MEM)/Rietveld方法分析高能同步辐射粉末衍射数据，广泛地研究了与相变相关的钙钛矿氧化物及其相关材料的电子电荷密度分布。立方相中的反铁电氧化物具有两个明显的结构特征，即B-O键上显著较低的共价键电子密度和O原子周围各向异性电荷密度分布。这些特征可以为理解为什么某些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.