Ferro- and Antiferroelectric Tetragonal Tungsten Bronzes for Capacitor Applications

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.