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

电场驱动的铁电体自发极化开关是一个基本过程，在铁电非易失性存储器等各种应用中发挥着重要作用。由于极化域通过电去极化场的长程相互作用，这个过程应该表现出很强的空间相关性。另一方面，偏振模式应该与局部随机晶体结构兼容。这两种类型的相关性都没有得到很好的理解，也没有包括到目前为止的任何统计概念的极化响应，虽然他们至关重要的影响所施加的电场的空间分布的统计特性，因此极化动力学。 本项目的目标是解释上述不可避免的相关性，并将随机局部电场的统计特性与多晶铁电体的微观和宏观性质联系起来。这包括微晶的局部相对称性、各向异性、织构和晶体取向的相关性。最后，该理论应允许推导的动态极化性能的系统从其结构特性。这一理论研究是基于最近开发的模型的非均匀场机制（IFM）的极化反转，并应进行密切合作与实验研究。后者将集中在一系列无铅铁电体从（K，Na）NbO 3家庭与不同的有序微观结构，包括单晶，外延薄膜，多晶薄膜和块状陶瓷。偏振转换将在九个数量级的时域内进行实验研究（体系统为10^（-6）至10^3 s，薄膜为10^（-9）至1 s）。为了阐明开关机制，电气测量将补充结构分析，如原位时间分辨高能X射线衍射。基于理论和实验研究之间的密切合作，理解和控制的极化动力学通过结构材料的性能应该达到。

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.