Polarization switching in lead-free ferroelectrics: statistical theory and experiments

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Electric field-driven switching of spontaneous polarization in ferroelectrics is a fundamental process that plays a central role in various applications, such as ferroelectric non-volatile memories. This process should exhibit strong spatial correlations due to long-range interactions of polarization domains through electric depolarization fields. On the other hand, polarization patterns should be compatible with locally random crystalline structures. Both sorts of correlations are not well understood and not included so far in any statistical concept of polarization response, though they crucially affect statistical properties of spatial distribution of the applied electric field, and consequently polarization dynamics. The goal of this project is to account for the aforementioned unavoidable correlations and to relate statistical characteristics of the random local electric fields with the microscopic and macroscopic properties of the polycrystalline ferroelectrics. This includes the local phase symmetry of crystallites, anisotropy, texture, and correlation of crystal orientations. Finally, the theory developed should allow for the deduction of the dynamic polarization properties of the system from its structure properties. This theoretical study is based on the recently developed model of Inhomogeneous Field Mechanism (IFM) of polarization reversal and should be performed in close collaboration with experimental investigations. The latter will focus on a series of lead-free ferroelectrics from the (K,Na)NbO3 family with a differently ordered microstructure comprising single crystals, epitaxial thin films, polycrystalline thin films and bulk ceramics. Polarization switching will be experimentally investigated over the time domain of nine orders of the magnitude (10^(-6) to 10^3 s for bulk systems and 10^(-9) to 1 s for thin films). In order to elucidate the switching mechanisms, the electrical measurements will be supplemented with structural analysis, such as in situ time-resolved high-energy X-ray diffraction. Based on the close collaboration between theoretical and experimental investigations, understanding and controlling the polarization dynamics through the structural material properties should be reached.