无铅超高居里温度菱方铁电薄膜的正/切应变效应研究

Due to the existence of ultrahigh Curie temperatures, lead-free rhombohedral ferroelectric films have plenty of applications in extreme high temperature environments, such as in aerospace engineering, nuclear plants, and geological exploration. Compared to the bulk counterpart, misfit strain has become a universally recognized method to tune the crystallographic structures, phases and related physical properties of epitaxial ferroelectric films. Although recent developments demonstrated that the normal strain is able to induce new structures ( and phases), the reported phase diagrams are not accurate enough due to the overlook of shear strain in lead-free ferroelectric films (i.e., BiFeO3 and LiNbO3) because of their ultrahigh Curie temperature associated with a phase transition from cubic to rhombohedral. Thus, it is desirable to systematically study the effect of normal strain and shear strain simultaneously on rhombohedral ferroelectric films, which enables explorations of a precise phase diagram and fundamental mechanisms responsible for the enhanced electronic properties..Using a combination of experiments and Landau-Ginzburg theory, we will detailedly study the effects of strains on these rhombohedral ferroelectric films. Besides the normal strain, shear strain is introduced to phenomenologically investigate its effect on the phase transition and related properties such as Curie temperature, piezoelectricity and ferroelectricity. Experimentally, epitaxial rhombohedral ferroelectric films will be grown on various substrates by laser molecular beam epitaxy method. High resolution X-ray diffraction will be performed to carefully determine the effects of normal and shear strains on the crystallographic structure, phase diagram and related electronic properties of rhombohedral films. By taking these steps, we hope that our results will enrich the strain engineering, particularly the shear strain, for advanced ferroelectric/piezoelectric properties in lead-free rhombohedral ferroelectric films with ultrahigh Curie temperature.

无铅超高居里温度菱方铁电与环境友好，在航空航天、核能、地质勘探等超高温环境下具有广泛应用。理解与调控界面应变效应是改善这类菱方铁电外延薄膜结构和提高性能的基础和关键。目前大部分工作只研究了正应变对这类薄膜的结构、相图和铁电等性能的影响。然而此类薄膜的界面中实际还存在另一重要的应变（即切应变），但却极少工作对此效应进行探索。本项目拟理论结合实验，研究无铅超高温菱方铁电薄膜BiFeO3和LiNbO3的正应变和切应变效应。应用朗道-金兹堡理论，探索正、切应变对这类铁电薄膜的相变、居里温度和铁电等性能的影响；采用激光分子束外延法在不同基底上制备菱方铁电外延薄膜，测量其结构、居里温度和铁电压电性能，研究正、切应变对其结构和性能的影响，揭示应变与薄膜结构和性能的关联。结合理论分析，获得更为精确的正/切应变诱导的相图，为进一步提高无铅超高温菱方薄膜的铁电等性能以及应变调控物性机理提供科学参考。

在本项目资助下，利用激光脉冲沉积技术，系统研究了在不同单晶基片上实现铁性薄膜（铁电PZT和铁磁LSMO）的外延生长。通过选取不同的基片和激光能量，改变外延体系中界面的失配应力，从而有效调制铁性薄膜的晶体结构和物理特性。最终获得具有超高居里温度（Tc > 600°C）的铁电PZT和铁磁LSMO薄膜，为极端宽温下航空航天，核工业探测和地质勘探等大大限制其工作环境和应用范围。比如，作为传感和探测器件，它无法满足很多高温条件下（特别是）的应用需求。