Domain Walls and Phase Boundaries in Ferroelectric Oxides

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9632989 Chen The proposed research will study the dynamics of ferroelectric domain walls and phase boundaries, which, in perovskites, are nearly atomically sharp with a thickness of the order of one to two atomic layer spacings. For such sharp interfaces, the mechanism of interfacial steps (in analogy to dislocation jogs and kinks) along with interactions of interfaces and obstacles (in analogy to dislocation pinning and bow-out) is central to the understanding of interfacial dynamics. These characteristics will be directly probed by measuring the migration energy, formation energy, and activation volume of the interfacial steps in expressly designed experiments under a stress field or an electrical field and will be complemented by additional structural and simulation studies that focus on the atomistics of the step mechanism for domain walls and phase boundaries. Three families of prototypical ferroelectrics with a perovskite structure, BaTiO3, Pb(Zr1-xTix)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN), along with their various compositional and microstructural modifications, will be investigated. For BaTiO3 and PZT type of ceramics, which have a sharp transition at the Curie temperature, Tc, the interest is primarily on the dynamics of domain walls below Tc. For PMN type of ceramics, which have a broader permittivity peak with the actual transition diffuse and well below the temperature of maximum permittivity (Tmax), the studies will be directed to the dynamics of phase boundaries below Tmax. Relaxation spectroscopy will be used to measure activation energy and activation volumes of mobile interfaces, which can be conveniently performed using polarization relaxation at small field and stress relaxation at large field. To lend further insight into the atomistic mechanisms that affect polarization reversal and interface mobility, synchrotron x- ray absorption spectroscopy (EXAFS) studies and Monte-Carlo lattice simulations will be conducted. These investigations will be carried out for both model ferroelectrics and their modifications to delineate the broader effects of chemistry, defect, and nanostructure that can influence interfacial dynamics by either providing obstacles to interface motion or by altering the mobility of the interface itself. The goal of the research is to provide a unified and detailed understanding, from the viewpoint of interface dynamics, of various interface-controlled ferroelectric, ferroelastic and piezoelectric phenomena that have broad implications to the performance and long-term reliability of ferroelectric ceramics as devices. %%% The goal of the research is to provide a unified and detailed understanding, from the viewpoint of interface dynamics, of various interface-controlled ferroelectric, ferroelastic and piezoelectric phenomena that have broad implications to the performance and long-term reliability of ferroelectric ceramics as devices. Three families of prototypical ferroelectrics with a perovskite structure, barium titantate, lead zirconium titanate, and lead magnesium niobate, long with their various compositional and microstructural modifications, will be investigated. These materials are used in many types of sensors, actuators, and electronic devices. ***