Ca3Ti2O7基陶瓷的杂化非本征铁电性及其调控

Hybrid improper ferroelectricity is a secondary ferroelectric ordering induced by the coupling of oxygen octahedral in-plane rotation and out-of-plane tilt in metal-oxides with the units of perovskite structure, it is expected to apply in the multiferroic materials with strong magnetoelectric coupling owing to its intrinsic characteristic for the electric-field control of magnetism, and it will greatly extend the connotation and denotation of ferroelectric physics. To clarify the critical behavior of ferroelectric phase transition and overcome the problem in the experimental work induced by its high coercive field, the present project focuses on the hybrid improper ferroelectricity in Ca3Ti2O7-based ceramics with Ruddlesden-Popper structure based on our previous work. To find out the detail transition path from high-temperature tetragonal paraelectric to room-temperature orthorhombic ferroelectric phase, the Ca3Ti2O7-based ceramics will be studied by in situ X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. Then the temperature dependence of electronic properties will be studied to clarify the physical nature of ferroelectricity in the present ceramics. Moreover, the variation of dielectric constant around the Curie temperature should be emphasized since it will significantly vary with the order of phase transition and the type of trigger mode in the improper ferroelectricity. Based on the previous results of first-principle calculations, the ferroelectric properties of the present ceramics can be controlled and optimized by tuning the average tolerant factor and the difference of tolerant factor of two perovskite layers, thus the ceramics with low coercive field and high polarization can be achieved by tailoring their components. Through the present work, it is expected to reveal the essential feature of hybrid improper ferroelectricity in Ca3Ti2O7-based ceramics, and this will promote the practical application of the present ceramics.

杂化非本征铁电性是指在具有钙钛矿结构单元的金属氧化物中由氧八面体面内旋转和面外倾侧耦合而诱导出的二阶铁电序，它有望在强磁电耦合多铁性材料中获得重要应用，并将极大地拓展铁电体物理学的内涵和外延。其面临的瓶颈在于铁电相变临界行为不明确及高矫顽场导致实验工作难以开展。因此，本项目拟在前期工作的基础上，着眼于具有Ruddlesden-Popper结构的Ca3Ti2O7基陶瓷的杂化非本征铁电性，通过原位结构分析手段研究其相变历程，确定其铁电相变机制；在分析其电学性能随温度变化规律，特别是介电常数在居里点附近演化规律的基础上，比较其与传统非本征铁电性的异同，揭示该铁电相变的物理本质；并在此基础上，通过成分裁剪调控平均许容因子及许容因子之差，实现Ca3Ti2O7基陶瓷铁电性能的优化与有效调控。通过本项目的实施，有望揭示杂化非本征铁电性的物理本质，并为Ca3Ti2O7基陶瓷杂化非本征铁电体的应用奠定基础。

杂化非本征铁电性是指在具有钙钛矿结构单元的金属氧化物中由氧八面体面内旋转和面外倾侧耦合而诱导出的二阶铁电序，它有望在强磁电耦合多铁性材料中获得重要应用，并将极大地拓展铁电体物理学的内涵和外延。Ca3Ti2O7是最典型也是最早被实验证实的杂化非本征铁电体，研究其铁电性起源和探寻调控优化铁电性能的有效途径具有重要的科学和实际意义。本课题首先利用中子衍射研究了4-1300K宽温范围内Ca3Ti2O7的晶体结构随温度演化规律，发现了氧八面体的倾转幅度随着温度变化出现反常现象。随后，利用实验和理论计算两种方式研究了不同位置离子置换对Ca3Ti2O7陶瓷的晶体结构与铁电性能的影响规律，并关注了氧八面体倾转幅度与铁电性能的关联。本课题发现了剩余极化值与氧八面体倾侧幅度成正比，而矫顽场与氧八面体的旋转幅度关联的规律，并在不同的Ca3Ti2O7基陶瓷中得到了证实。因此，本课题探寻出调控和优化Ca3Ti2O7基陶瓷的有效途径，即增强氧八面体的倾转幅度而抑制氧八面体的旋转幅度即可获得高剩余极化、低矫顽场的铁电性能，这为该材料的实用化奠定坚实的基础。同时，根据该领域发展的新趋势，适度增加了单相多铁性及新型杂化非本征铁电体的研究，取得的重要进展、并促进了Ca3Ti2O7基陶瓷本身的研究。

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