Modelling and Designing Ferroelectrics with Defects and in Two-Dimensional Forms

这项理论研究将建立一个项目，旨在研究铁电材料中的缺陷，并揭示它们对局域和基态结构、相变、介电和机电响应的影响；确定和了解铁电薄膜的特性；以及设计具有最佳和/或原始特性的新型铁电材料。研究计划与学生的教育经验相结合。研究目标将通过开发和/或使用以下最先进的和从头计算的数值工具来实现：精确的第一原理技术，包括总能量计算、声子谱计算、现代极化理论和电场效应；有效的哈密顿方法，通过现实地模拟有限温度下钙钛矿和/或铁电薄膜中的缺陷，扩大第一原理计算的范围；以及逆方法，允许高效地设计性能得到改善的材料。阿肯色州与其欧洲合作伙伴之间的合作活动将允许在预测和测量之间进行仔细的比较，这对于全面了解要研究的系统以及改进要开发的数值工具(如果需要)是很重要的。许多缺陷和薄膜及其对性能的影响将被研究。例如，弛豫体中的化学有序纳米区域，BaTiO_3中的氧空位，Pb(Sc，Nb)O_3中的铅空位和B位上的铅错位，Pb(Zr，Ti)O_3中的La掺杂，以及(寄生的)焦绿石相。其他例子是BaTiO3薄膜的相变顺序和性质(即正常与扩散)与其厚度、表面终止、生长方向、衬底、压力、机械边界条件和电场的依赖关系。重点将着重于确定某些块体材料中已知的一些显著特征--如KTaO_3中的量子效应，Pb(Zr，Ti)O_3中的单斜相和反铁扭曲位移，以及内电场和原子有序对Pb(Sc，Nb)O_3性质的影响--从块体到薄膜的演化。铅(Sc，Nb，Ti)O_3薄膜中的(原子和薄膜相关)自由度也有望发现新的压电和介电性能的现象和优化。由于所使用的技术和所研究的系统的多样性，人们将对含有缺陷的钙钛矿结构和铁电薄膜有更广泛的了解。此外，为了建立一个网络，作为相关机构之间未来合作和交换学生的基础，合作努力也有可能导致实现具有改进和/或新功能的设备，这将积极影响生活质量和提高安全性。一个网站将报告一个与调查结果相关的数据库，并将提供在项目期间开发的代码。此外，学生将接受所有级别的培训。这个理论研究项目研究新的铁电材料和由这些材料形成的薄膜。铁电材料的独特之处在于，当满足某些条件时，它们能够产生自发极化。这使得这些材料在各种应用中特别有用。拟议中的计划与欧洲的组织进行了广泛的国际合作。学生将参与所有级别的研究。*

This theoretical research will build a program aimed at investigating defects in ferroelectrics and revealing their effects on local and ground-state structures, phase transitions, dielectric and electromechanical responses; determining and understanding the properties of ferroelectric thin films; and, designing new ferroelectrics with optimal and/or original properties. The research program is integrated with the educational experiences of students.The research objectives will be achieved through the development and/or use of the following state-of-the-art and ab initio numerical tools: accurate first-principles techniques, including total energy calculations, computation of phonon spectrum, modern theory of polarization and electric-field effects; effective Hamiltonian approaches that will extend the reach of first-principles calculations by realistically mimicking defects in perovskites and/or ferroelectric thin films at finite-temperature; and the inverse method that allows an efficient design of materials with improved properties. Collaborations with well-known European groups and industries having a vital experimental or applied program in ferroelectrics will be strengthened.The cooperative activities between Arkansas and its European partners will allow a careful comparison between predictions and measurements, which is important to fully understand the systems to be investigated, and to refine the numerical tools to be developed (if needed). Many defects and thin films, and their effects on properties, are going to be investigated. Examples include chemically-ordered nanoscale regions in relaxors, oxygen vacancies in BaTiO3, Pb vacancies and Pb misplaced on B sites in Pb(Sc,Nb)O3, La dopants in Pb(Zr,Ti)O3, and (parasitic) pyrochlore phases. Other examples are the dependencies of the phase transitions sequence and nature (i.e., normal vs. diffuse) of BaTiO3 thin films on their thickness, surface termination, growth orientation, substrate, pressure, mechanical boundary conditions and electric fields. Particular emphasis will be put on determining how some striking features known to occur in some bulk materials, - e.g., quantum effects in KtaO3, monoclinic phase and antiferrodistortive displacements in Pb(Zr,Ti)O3, and the role of internal electric field and atomic ordering on properties of Pb(Sc,Nb)O3 - evolve when going from bulk to thin films. New phenomena and optimization of piezoelectric and dielectric properties are also expected to be discovered by playing with the (atomic and film-related) degrees of freedom in Pb(Sc,Nb,Ti)O3 thin films.A broad knowledge of perovskites containing defects, and of ferroelectric thin films, will be gained thanks to the diversity of techniques to be used and the variety of systems to be investigated. In addition, to build a network that will be the basis for future collaboration and exchange of students between the involved institutions, the collaborative efforts have also the potential to result in the realization of devices with improved and/or new functionalities, and that will positively affect quality of life and improve safety. A website will report a database related to the findings, and will provide the codes to be developed during the project. In addition, students will be trained at all levels.%%%This theoretical research program investigates new ferroelectric materials and thin films formed from these materials. Ferroelectric materials are unique in their ability to create a spontaneous polarization when certain conditions are met. This makes the materials particularly useful in a variety of applications. The proposed program has an extensive international collaboration with groups in Europe. Students will participate in the research at all levels.***