Dielectrics by design: chemical control of dipole behaviour

There is considerable drive to develop novel ferroelectric (piezoelectric) materials for both high permittivity applications (e.g. capacitors) and lead free alternatives to PbZxTi1-xO3 (PZT). My talk will outline two of our current areas of activity in this broad area of materials development and characterisation.
The first will describe our recent work on oxides with the tetragonal tungsten bronze structure where we have related the local and average chemical structure to the dynamics and stability of dipoles. We have identified two broad types of behaviour in the materials studied: firstly that the macroscopic tetragonal crystal strain determined by the B-site cation size determines the dipole stability in the active c-axis; secondly that local strain gradients induced by judicious choice of A-cation can also be employed to control the dipolar behaviour. By careful and selective doping it is possible to develop materials with dipolar behaviour ranging from disordered (relaxor dielectric) to fully ordered (ferroelectric).
Secondly, rare earth doping has been demonstrated as an effective way of mitigating both the thermodynamic metastability and leakage current in the widely studied multiferroic perovskite BiFeO3. We have investigated the structure-property relations in La- and Nd-doped BiFeO3 using a combination of temperature dependent powder neutron diffraction and immittance spectroscopy. The structural variation in these materials occur due to a changing balance between magnetic ordering and geometry optimization to fulfill bonding requirements, and has important ramifications for the dielectric properties.