Determining the atomic structure of incommensurate antiferroelectrics based on La-doped Pb(Zr,Ti)O3

锆钛酸铅(Pb(Zr，Ti)O_3)体系显示出一系列令人着迷的结构和行为，但有一个共同的特征，即所有的组成在室温下由于氧阴离子和金属阳离子的反平行位移而表现出永久的电极化。在组成范围的PbZrO_3端，电偶极子排列成反平行极化的条纹，导致零净极化，这被称为反铁电态。与此相反，对于Pb(Zr[0.9]，Ti[0.1])O_3，极化都位于同一方向，导致有限的永久宏观极化--铁电态。仅在后一种成分中掺杂2-4%的La就会使其进入一种略微混乱的状态，非常接近铁电和反铁电有序的边缘。虽然众所周知，这种状态的晶体结构具有一个大的晶胞，这是不相称的(即，它不完全堆积成简单的整数个原子堆积)，但这种结构的细节根本不被很好地理解。原因很简单：它很大，而且没有完美的有序(之前的研究表明，它经常偏离完美的有序)，因此像X射线或中子衍射这样的技术将会遇到困难。虽然可以从传统的电子显微镜和衍射中推断出一些信息(申请人已经完成了这项工作)，但解决结构问题的最直接方法是能够看到所有原子的位置。由于像差校正电子显微镜的进步，这现在是可能的。最近的发展使得补偿所有电磁透镜中存在的缺陷成为可能，这使得我们现在能够分辨远低于1+的物体--这是分辨原子的合适尺度。Jlich的项目合作伙伴在将其应用于材料方面处于世界领先地位，在对钙钛矿氧化物进行此类研究以及通过成像这些结构中的氧和金属阳离子来测量电极化方面拥有独特的经验。该项目将允许申请人以他以前的无公度反铁电体经验前往Jlich，并与他们合作，以亚+ngstrm分辨率成像这些迷人的材料，并结合数据处理和图像模拟，使我们能够确定整个单元电池的氧和阳离子位移。除了解决这一有趣的相的结构外，它还将使我们能够更好地理解它与理想的PbZrO_3反铁电相和Pb(Zr[0.9]，Ti[0.1])O_3的菱方铁电相之间的关系，并为未来研究反铁电相和铁电相之间的场致相变奠定基础。

The lead zirconate titanate (Pb(Zr,Ti)O3) system displays a fascinating range of structures and behaviours, but with the common feature that all compositions contain exhibit permanent electric polarisation at room temperature as a result of antiparallel displacements of the oxygen anions and the metal cations. At the PbZrO3 end of the composition range, the electric dipoles are arranged in stripes of antiparallel polarisation resulting in zero net polarisation, this is referred to as an antiferroelectric state. In contrast to this, for Pb(Zr[0.9],Ti[0.1])O3, polarisation all lies along the same direction resulting in a finite permanent macroscopic polarisation - a ferroelectric state. Just doping this latter composition with 2-4% La puts this into a slightly confused state, very much on the edge between ferroelectric and antiferroelectric ordering. Whilst it is well known that the crystal structure for this state has a large unit cell, which is incommensurate (i.e. it doesn't quite stack up as being made of a simple whole number of atomic stackings), the details of this structure are not at all well understood. The reasons for this are straightforward: it is big and not perfectly ordered (previous studies show frequent deviations from perfect order) and thus techniques like diffraction with X-rays or neutrons will have difficulties. Whilst some information can be inferred from conventional electron microscopy and diffraction (which has already been done by the applicant), the most straightforward way to solve the structure would be to be able to see where all the atoms are. This is now possible due to advances in aberration corrected electron microscopy. Recent developments have made it possible to compensate for the imperfections present in all electromagnetic lenses and this now allows us to resolve objects well below 1 + - a suitable scale for resolving atoms. The project partners at Jlich are world leaders in applying this to materials and have particular experience with doing such studies on perovskite oxides and in measuring electrical polarisation from imaging the oxygen and the metal cations in these structures. This project will allow the applicant with his prior experience of incommensurate antiferroelectrics to travel to Jlich and collaborate with them on imaging these fascinating materials at sub-+ngstrm resolution and in combination with data processing and image simulation to enable us to be able to determine the oxygen and cation displacements across the unit cell. As well as solving the structure of this interesting phase, it will also enable us to better understand its relationship to both the ideal antiferroelectric phase of PbZrO3 and to the rhombohedral ferroelectric phase of Pb(Zr[0.9],Ti[0.1])O3, and will prepare the ground for future studies of field induced transformations between antiferroelectric and ferroelectric phases.