掺杂与应变协同调控BiFeO3外延薄膜的正交-菱方-四方相变研究

应变调控BiFeO3薄膜的四方(T)、菱方(R)、正交(O)等相变及准同型相界(MPB)的研究已成为挖掘其新奇物性的重要手段。目前的研究集中在压应变调控R-T相变及相界上，衍生了一系列奇特物性(强铁电性、压电增强、磁性增强等)。然而，拉应变基片的缺乏导致了R-O相变及R/O MPB研究的滞后(理论上预测具有更加优异性能)。对R相纯BiFeO3来说，可能具有最优物性的三相共存态及multi-MPB则更难实现。我们的前期研究获得了La掺杂的O相BiFeO3。由此，我们提出掺杂与应变协同调控O-R-T三相连续相变。本项目拟采用PLD生长高质量薄膜，研究不同掺杂和应变作用下的相结构演变机制，以确定相变临界厚度，进而构建O/R、R/T MPB，利用应变梯度效应实现三相共存及multi-MPB，并探索相关新颖物性。本项目将建立BiFeO3的应变相图，并绘制其结构-物性关联图景，最终获得高性能材料。

Strain engineering is a powerful pathway to induce tetragonal-like (T), rhombohedral-like (R), orthorhombic(O) phase transitions and create morphotropic phase boundary (MPB) in BiFeO3, playing a key role and providing a new route to explore the potential physical properties. Recently, the compressive strain driven R-T phase transitions and R/T MPB have been widely studied, revealing plenty of excellent physical properties, such as enhanced ferroelectricity, piezoelectricity and magnetism, etc. Theoretical studies suggest much greater properties in R/O MPB, however, few reports have focused on tensile strain driven R to O phase transitions and R/O MPB due to the lack of substrates that can provide strong tensile strain. For pure BiFeO3, it is more difficult to achieve triple phase coexistence and multi-MPB which may own the most excellent physical properties. Therefore, more effectively approach is desired to drive the phase transitions and MPB...In our previous study, BiFeO3 with O phase has been successfully obtained by La doping. Inspired by this, here we propose to realize the O-R-T phase transitions in BiFeO3 epitaxial thin films by the coordinate effects between chemical doping and strain engineering. We will optimize the PLD growth conditions to grow high quality (La-)BiFeO3 epitaxial thin films on various substrates. The structure evolution mechanism with doping and strain effects will be investigated to obtain the critical thickness of O, R and T phase and create the O/R, R/T MPB. Combining with the strain gradient effect, triple phase existence and multi-MPB will be realized. Meanwhile, we will explore the related novel physical properties. We aim to establish the strain phase diagram of BiFeO3 and study the relationship between the crystal structure and physical properties to obtain high performance multiferroic materials.