Soft chemical routes to novel magnetoelectric materials

我们越来越多地看到，将有用的、迷人的现象结合成一种单一的、多功能的材料，正在推动新型先进材料的发展。最有趣的是那些相互作用而产生合作效应的属性。磁电多铁性材料同时具有铁磁性（包含南北两极的排列磁偶极子）和铁电性（包含由正电荷和负电荷组成的排列电偶极子），这意味着它们在单相内显示出可以通过外加磁场重新定向的自发磁化和可以通过外加电场重新定向的自发电极化。除了对其基本物理性质的科学兴趣外，新型多铁性材料不仅提供了铁磁性和铁电材料的所有潜在应用，而且还为一系列其他多功能应用打开了大门。其中最令人兴奋的是能够用电场控制相的磁性，使依赖磁性的技术更加节能。然而，这些材料已被证明是极具挑战性的合成，因为一种成分的铁性质的存在往往排除了另一种成分的出现。因此，研究的重点是开发可用于设计这些材料的新方法。钙钛矿相是研究这些性质的理想体系，因为其结构灵活，可以容纳一系列元素。立方钙钛矿的通式为ABX3，其中a位离子占据晶格的四角，b位离子占据八面体的中心，而6个x离子位于顶点。最近，由于BX6八面体在钙钛矿结构中普遍存在，并且可以通过仔细的化学取代来控制，因此人们已经做出了相当大的努力来利用它们的协同旋转和倾斜。最重要的是，八面体旋转与材料的磁性和电子特性强烈耦合。一种新的机制，被称为“三线性耦合机制”，利用层状钙钛矿的旋转模式，已经被研究作为在这种材料中诱导铁电性的策略。该项目的总体目标是合成新型磁电多铁性钙钛矿材料，利用三线性耦合机制诱导铁电和软合成路线，以获得传统陶瓷固态方法（如极高温度）无法获得的相。更具体地说，将合成一系列扭曲的层状氟化物-钙钛矿相。现有的关于层状氟化物相的文献很少，因为它们的氧化物类似物往往更稳定，因此更容易合成。因此，新的基于溶液的合成方法将与固态技术相结合，以获得新的相。这些材料将使用非常高强度的x射线和中子进行研究，这是必要的，因为所研究的八面体旋转和倾斜与其他材料相比很小。该项目属于EPSRC物理科学研究领域，由牛津大学物理系的Stephen Blundell教授共同监督。

More and more, we are seeing that combining useful and fascinating phenomena into a single, multifunctional material is driving the development of new advanced materials. Most interesting are properties that interact mutually to result in cooperative effects. Magnetoelectric multiferroic materials are simultaneously ferromagnetic (containing lined up magnetic dipoles of north and south poles) and ferroelectric (containing lined up electric dipoles made of positive and negative charges), meaning that they display a spontaneous magnetisation that can be reoriented by an applied magnetic field, and a spontaneous electric polarisation that can be reoriented by an applied electric field, within a single phase. Besides scientific interest in their fundamental physical properties, novel multiferroic materials not only offer all the potential applications of ferromagnetic and ferroelectric materials, but also open the door to a range of other multifunctional applications. The most exciting of these is the ability to control the magnetic properties of a phase with an electric field, making technology which relies of magnetism more energy efficient. However, these materials have proven to be extremely challenging to synthesise, since the presence of one constituent ferroic property often precludes the onset of the other. As such, research has focussed on developing new approaches that can be used to design these materials. The perovskite phase is an ideal system within which to study these properties, as the structure is flexible and can accommodate a range of elements. The cubic perovskite has the general formula ABX3, where the A-site ion occupies the corners of the lattice and the B-site ion occupies the centre of an octahedron, whilst six X-ions are positioned on the vertices. Recently, considerable effort has been made to exploit the cooperative rotations and tilts of the BX6 octahedra since they are ubiquitous in the perovskite structure and can be controlled through careful chemical substitution. Most importantly, the octahedral rotations couple strongly to the magnetic and electronic properties of the material. A novel mechanism, coined the 'trilinear coupling mechanism', which takes advantage of the rotation patterns of layered perovskites, has been investigated as a strategy to induce ferroelectricity in such materials. The overarching aim of this project is to synthesise novel magnetoelectric multiferroic perovskite materials, exploiting the trilinear coupling mechanism to induce ferroelectricity and soft synthetic routes to obtain phases that would not be accessible using traditional ceramic solid-state methods, such as extremely high temperatures. More specifically, a series of distorted layered fluoride-perovskite phases will be synthesised. Existing literature on layered fluoride phases is scarce, since their oxide analogues tend to be more stable and as such, easier to synthesise. Because of this, novel solution-based synthetic methods will be employed in combination with solid-state techniques to obtain new phases. These materials will be studied using very high intensity x-rays and neutrons, which are necessary as the octahedral rotations and tilts being studied are small compared to the rest of the material.This project falls within the EPSRC Physical sciences research area, and is co-supervised by Professor Stephen Blundell from the Department of Physics, Oxford University.