Ferro- and Antiferroelectric Tetragonal Tungsten Bronzes for Capacitor Applications

铁电性定义为在外加电场作用下可逆的自发极化。铁电（FE）材料也表现出压电性：材料在外加电压下机械扭曲，如果施加机械压力，也会产生电荷。因此，铁电（FE）材料被广泛用于各种电子设备，包括电容器、电光开关、非易失性存储芯片（在没有电源的情况下保留数据的非易失性存储器）和许多压电（PE）换能器和传感器。FE随机存取存储器（fram）使用两个电压可切换的极化状态（+ve，“上”和-ve，“下”）来表示二进制代码，PE设备将电场转换为机械位移（用于喷油器，喷墨打印头，扬声器），机械能转换为电荷（气体点火器，麦克风），或两者都转换（医学超声，声纳）。反铁电材料不太常见，但由于高压或器件小型化，在高场应用中作为多层陶瓷电容器（mlcc）具有潜在的应用前景。目前全球mlcc市场每年超过4万亿个零件，需求的增长超过了生产。电子工业不断寻求新的铁电和反铁电材料。目前使用最广泛的两种铁电材料（BaTiO3）和反铁电材料（PbZrO3）都是钙钛矿，由于结构简单，并且能够容纳元素周期表上几乎所有的元素，对新材料的探索很大程度上停留在这一家族类型中。本项目旨在研究具有四方钨青铜（TTB - a '2A"4B'2B"8O30）结构的一系列材料的组成和晶体结构对介电和（反）铁电性能的影响。TTB结构与研究最广泛的钙钛矿（ABO3）密切相关，因为它由BO6八面体组成的角共享网络（B通常是d-嵌段金属），较大的阳离子（s-嵌段或镧系元素）占据八面体之间的通道。将研究一系列TTB化合物的结构、介电和铁电性质，以确定其在高压电容器中的潜在用途；这些研究是在我们研究小组已经开展的初步工作的基础上进行的。该项目将涉及：（固态）合成工作，包括陶瓷加工；用x射线和中子衍射及全散射技术测定晶体结构；电学表征使用一系列技术，如介电和阻抗光谱，极化场和热激退极化电流测量。

Ferroelectricity is defined by a spontaneous polarisation which is reversible under an applied electric field. Ferroelectric (FE) materials also exhibit piezoelectricity: the material mechanically distorts under an applied voltage and also, if mechanical pressure is applied, an electric charge is generated. Ferroelectric (FE) materials are therefore used in a wide range of electronic devices including capacitors, electro-optic switches, non-volatile memory chips (a non-volatile memory retains data in the absence of power) and a number of piezoelectric (PE) transducers and sensors. While FE random access memories (FRAMs) use the two voltage-switchable polarisation states (+ve, "up" and -ve, "down") to represent binary code, PE devices convert electric fields to mechanical displacements (utilised in fuel injectors, inkjet print heads, loudspeakers), mechanical energy to electrical charge (gas ignitors, microphones), or both (medical ultrasound, sonar). Antiferroelectric materials are less common, but have potential application as multi-layer ceramic capacitors (MLCCs) in high field applications either as a result of high voltage or device miniaturisation. The current annual worldwide market for MLCCs is in excess of 4 trillion parts per annum, with growing in demand outstripping manufacture. The electronics industry is constantly seeking new ferroelectric and antiferroelectric materials. Currently the two most widely used ferroelectric (BaTiO3) and antiferroelectric (PbZrO3) materials are both perovskites and the quest for new materials has largely stayed within this family type because of the simplicity of structure and its ability to accommodate nearly every element from the periodic table.The aim of this project is to investigate the influence of composition and crystal structure on the dielectric and (anti-)ferroelectric properties of a range of materials with the tetragonal tungsten bronze (TTB - A'2A"4B'2B"8O30) structure. The TTB structure is closely related to the most widely studied perovskite (ABO3) in that it consists of a corner-sharing network of BO6 octahedra (B is typically a d-block metal), with larger cations (s-block or lanthanide) occupying the channels between octahedra. The structural, dielectric and ferroelectric properties of a range of TTB compounds will be investigated for their potential use in high voltage capacitors; these studies follow on from preliminary work already carried out in our research group. The project will involve: (solid-state) synthetic work, including ceramic processing; crystallographic structure determination using both x-ray and neutron diffraction and total scattering techniques; and electrical characterization using a range of techniques such as dielectric and impedance spectroscopy, polarization-field and thermally stimulated depolarization current measurements.