Structure-Property-Chemistry Relationship in Ferroelectric Bronzes: Material Science Issues in Tilted Oxygen Octahedra

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The primary goal of this research project is to develop underlying materials science in solid solutions of tungsten bronze (TB) structure and to enrich the present general understanding of the oxygen octahedral ferroelectric materials and relaxors, thereby enhancing the capability to design, select and modify electronic ceramics and single crystals for various device applications. Particular research topics include (a) the construction of structure-field-maps for TB compositions; (b) crystallographic structure studies by single crystal x-ray diffraction; (c) understanding the cation/vacancy distribution and order on relaxor characteristics and structural properties; and (d) the effect of thermodynamic fluctuation of large cations on low temperature dielectric dispersions. The project will consist of critical experiments and analysis that clarify the underlying structure-property-chemistry relationship in the ferroelectric TB oxide family. A structure-field-map for known TB compositions that summarizes crystallographic phase stability over ionic radii and pressure/temperature conditions will be constructed. Morphotropic phase boundary (MPB) systems can therefore be charted and predicted. Single crystal diffraction will be conducted to obtain cationic displacement details in over a broad temperature range. A selected-electric-field-condition single crystal x-ray diffraction and structure refinement system will be designed and built. It will be the first of its kind to allow the study of cation displacement under an electric field. Quenching and annealing studies on single crystals of near MPB composition with filled or partially filled A-site will be carried out to clarify the effect of cation site-preference and the vacancy distribution. Low temperature phase transition/relaxation phenomena will be studied by structural analysis (low temperature XRD and neutron diffraction), along with a dynamic pyroelectric measurement. The project will provide students from PSU and from Lincoln University (an HBCU) with an intellectual and challenging research project with access to both university and government lab facilities. Ferroelectric materials of the tungsten bronze structure encompass a large family of solid solution systems with great potential in various electronic, optical and electro-optic applications. The understanding that will result from this project will take materials scientists toward a more unified understanding on the structure-property-chemistry relations in deformed and tilted oxygen octahedral structures, and allow the possibility of intelligent selection of the materials for technologically important device applications. ***