Aurivillius型单相多铁材料晶格畸变及磁结构的中子衍射研究

Multiferroic materials have attracted considerable attention due to their potential applications in multifunctional devices. And the simultaneous occurrence of both ferroelectric and magnetic orders in a single phase material is fascinating. Aurivillius oxides are well known for their excellent ferroelectric properties and provide more magnetic structures as candidates of single-phase multiferroic materials. However, the existences of lattice distortion modes and magnetoelectric coupling in Aurivillius oxides are still controversial. Neutron diffraction technique is considerably suitable to the study of these materials in bulk polycrystalline form due to the large variation in the scattering factors for the elements involved; and this technique allows precise and meaningful location of oxygen atom positions and magnetic structure determination. We plan to combine Rietveld refinements of x-ray and neutron powder diffraction data to understand the subtle lattice distortions in 3-layer Aurivillius oxides that yield off-centering displacements by substitutions, and investigate the magnetic structures to explain the magnetoelectric coupling in these oxides.

单相多铁材料中的电极化、磁性具有丰富的物理机制，其铁电与磁性的耦合与调控一直是工业应用和基础科学面临的关键课题。本项目以奇数层n=3的单相Aurivillius型化合物为研究对象，通过改变组成元素优化材料多铁性能，针对其结构相变中的疑点，利用中子散射方法在区分近邻元素占位、确定氧等轻原子在晶胞中的位置及磁结构测量方面的独特优势，与X射线衍射相结合，研究晶格畸变及磁结构；通过与铁电、磁性等测试结果比对，分析其结构与铁电、磁性质的内在关系。本研究对优化材料合成方法、阐明多铁磁电耦合机制具有重要意义。

Aurivillius型化合物所具有的独特的天然层状结构为铁电、磁性能的同时实现提供了得天独厚的母体。Aurivillius型化合物的晶格畸变是研究难点。过去三年里，我们利用中子衍射方法的独特优势，结合X射线衍射方法研究n=3的Aurivillius型化合物Bi4Ti3O12及其Fe掺杂系列样品结构畸变及磁结构。包括：摸索熔盐法制备Aurivillius型化合物的实验条件，成功制备出n=3的Bi4Ti3O12及掺杂Fe元素的Bi4Ti(12-3x)/4FexO12系列样品，利用扫描电子显微镜及X射线衍射对样品形貌及微观结构进行观测，并展开磁性测量。XRD数据结果表明，在未掺杂时，为纯相的Bi4Ti3O12，而在Fe掺杂量增加至x=0.8时，则变为n=4的纯相Aurivillius型化合物。磁测量显示掺Fe样品有弱的反铁磁性，可以用修正的居里外斯定律（χ=χ_0+C/(T+θ)）描述。χ_0值反映了弱的非顺磁贡献，其源于Fe自旋之间的反铁磁交换作用。中子衍射射方法在区分近邻元素掺杂的阳离子占位、确定氧等轻原子在晶胞中的位置测量方面有着独特的优势，因此我们利用中子束流时间，针对上述系列样品进行中子衍射实验研究其微观结构，分析Fe原子占位，分析其中氧八面体的晶格畸变，探索其从B2cb空间群到A21am空间群的转变，同时探索Fe替代对晶格畸变及物理性质的影响，发现其主要畸变是沿a方向（垂直于层方向c轴）的极性离子位移，此外还包括氧八面体复杂的倾斜畸变。