In Search of Ferroelectricity in Antiferroelectric Lead Zirconate

Since the prediction of antiferroelectricity and the subsequent discovery of PbZrO3, the description of antiferroelectric behaviour in materials has been constantly modified to account for the latest properties identified in antiferroelectric materials. Adding to this refinement, the current study considers Lead Zirconate (PbZrO3) as a prototypical antiferroelectric and through it, aims to understand the origin of antiferroelectricity, and the possibility of localised ferroelectric behaviour inside the material. The observation and control of the occurence of such localised ferroelectricity is attempted and the mechanical behaviour of such ferroelectric structures is simulated. Antiferroelectrics are defined as materials which undergo a phase transition from one centrosymmetric phase to another while being accompanied by a dielectric anomaly at the transition. The origin of antiferroelectricity as observed in the prototypical antiferroelectric PbZrO3, involves multiple lattice instabilities with interactions that remained only partially deciphered. The current study provides for an explanation of the lattice dynamics occuring in PbZrO3 through results obtained from a combination of scattering techniques such as Inelastic XRay, Thermal Diffused and Brillouin scattering. The primary structural instability associated with the antiparallel lead displacements is seen to be coupled with the ferroelectric order parameter, and to the order parameter related to the oxygen octahedral rotations. On the appearance of the structural order parameter, these interactions result in the formation of the antiferroelectric phase of PbZrO3 as known previously. The premises for such interactions and their validations are provided, while explaining all aspects associated with the antiferroelectric phase transition occuring in PbZrO3 including the dielectric anomaly. One significance of the scenario with competing order parameters lies in the possible appearance of the subdued order parameters in regions where the primary order parameter is absent. The conditions for the disappearance of this order parameter in the case of PbZrO3 is discussed, and structural domain boundaries are shown to be potential regions for the local observation of otherwise subdued ferroelectricity. Aimed at the observation of localised ferroelectric structures in an otherwise antiferroelectric material, growth of epitaxial thin films of PbZrO3 is undertaken using Pulsed Laser Deposition. The growth parameters are varied to control the crystalline orientation and the defect concentration in the films through the control of the interfacial strain. In this process, a technique for the tunable variation of the epitaxial strain using a single composition buffer layer has been documented. The antiferroelectric films are then utilised for the observation of regions of disrupted symmetry, for the observation of localised ferroelectricity. The distribution of such regions in the thin films, as well as the anomalous interaction of different types of domain walls with crystalline defects are observed and analysed. Alongside, the response of such localised ferroelectric structures to external electric fields is simulated. The conditions necessary for sufficiently large displacements of such structures, large enough for potential observation using Scanning Probe Microscopy techniques, have been listed.

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