Local Structure and Ferroelectric Relaxor Behavior in Dion-Jacobson Layered Perovskites 

Description

Ferroelectric materials have a spontaneous electrical polarization which can be furthermore switched under an applied electric field. Ferroelectrics have fascinated solid-state physicists and materials scientists for decades because of their unique dielectric, piezoelectric, pyroelectric and electrooptic properties, ferroelectrics. They are immensely important for various technologicalapplications, such as capacitors, sensors and actuators, non-volatile memories, energy harvesting, and telecommunications. Most well-known ferroelectrics are complex oxides with the general formula ABO3and perovskite crystal structure. Driven by technological demands for application of ferroelectrics under more extreme conditions, there is an increased interest for exploration of ferroelectricity in complex oxides of other structure types. In this regard, layered perovskite phases have shown great potential due to their higher TC. The crystal structure of layered perovskites consists of perovskite-like blocks, interspersed with layers of A' or A'-O, where A' is a third cation. Particularly, Dion-Jacobson phases with formula of A'[An-1BnO3n+1] have gathered much attention due to their theoretically predicted high polarization, ultrahigh TC (~ 800-1000 °C) and low dielectric losses. Nevertheless, for potential technological application of Dion-Jacobson ferroelectrics, it is desirable that their various functional properties are enhanced, includingdielectric and piezoelectric properties.In the past, it was shown for ferroelectrics with perovskite as well as Aurivillius layered perovskite structures that the functional properties could be enhanced with the formation of a relaxor phase. In contrast to normal ferroelectrics, which exhibit a long-range polar order and a sharp paraelectricto-ferroelectric phase transition, relaxors exhibit nanoscale polar order and a broad phase transition over large temperature range. Relaxor ferroelectrics can be induced through the introduction of nanoscopic compositional and structural heterogeneities, which are created by suitable solidsolution substitutions at different atomic sites. The current proposal aims to develop new compositions of relaxor ferroelectrics based on Dion-Jacobson phases, which has not been explored so far.In order to undertake a rational approach for development of relaxors with Dion-Jacobson structure, it is important to develop a clear understanding of the local atomic correlations. Our preliminary study in this area using pair distribution function method indicates that interaction between oxygen octahedral tilting and polar atomic displacement modes in Dion-Jacobson compounds lead to local structural deviations from the long-range average crystal structure. Based on this initial insight, it is proposed that nanoscale polar order, and hence relaxor phase, can be induced in Dion-Jacobson compounds by suitable chemical substitutions at different atomic sites (9A/B/ A'), which locally alters the octahedral tilting modes and Cation-Oxygen bond covalency. We have tested this hypothesis by conducting preliminary studies which demonstrated that targeted chemical substitutions in Dion-Jacobson compounds can indeed be used to alter local structural order, induce relaxor-like behavior and enhancement in properties.Here, I propose to undertake a systematic study to map the effects of various solid-solution substitutions at the different atomic sites of A'[An-1BnO3n+1] compounds to induce relaxor behavior. The targeted atomic substitutions will be selected based on generic crystal chemical principles, as further elaborated in the proposal. The ultimate objective is to develop new type of ferroelectrics for application in extreme conditions.