Multi-scale structural response of the lead-free perovskite-type ferroelectric solid solution (1-x)Na0.5Bi0.5TiO3-xBaTiO3 to high pressures

复杂钙钛矿型（ABO 3）铁电体是关键的功能材料之一，因为它们能够将各种类型的外力场转换为电信号。目前，最重要的工业铁电固溶体是PbZr 1-xTixO 3（PZT），其表现出成分驱动的相变，称为准同型相界（MPB），在该相界处，介电、压电、热电和光电性能增强。（1-x）Na_（0.5）Bi_（0.5）TiO_3-xBaTiO_3（NBT-xBT）固溶体是一种非常有前途的无铅替代物，因为它与PZT类似，具有几乎与温度无关的MPB。然而，尽管存在宏观极化，但NBT-xBT的平均结构在MPB处似乎是伪立方的，这表明丰富的纳米级结构不均匀性和初期的铁性聚集。因此，需要在不同的外部刺激（温度，弹性应力/压力，电场），可以提高铁电秩序下的全面的原位结构分析，以揭示在碱金属铋基系统的材料性能的内在结构特征。在不同的温度和电场下进行了大量的研究，但令人惊讶的是，NBT-xBT的高压结构分析很少，而且大多限于端元。本项目旨在填补这一知识空白。我们将通过拉曼光谱、同步加速器X射线漫散射和高精度内部X射线衍射研究NBT-xBT单晶在MPB（x = 0，0.013，0.048，0.053，0.074）上不同长度尺度下压力诱导结构转变的组成依赖性，最高可达~21 GPa，使用金刚石砧室技术。声子的压力演化以及X射线漫散射和状态方程之间的比较，将有可能确定弹性不稳定性的原子起源和平坦的本地潜力，支持属性增强。将确定体积压缩率以及声子Grüneisen参数作为组合物的函数。此外，各向异性应力与静水压力的影响对局部结构和动力学的NBT-xBT单晶横跨MPB的基础上的拉曼散射非静水条件下高达~5.6 GPa进行分析。预期的结果应该提供对NBT-xBT结构的更深入的了解，并更好地揭示基于Pb和无铅系统之间固有的纳米级结构差异，这反过来可以帮助设计具有与富Pb材料相当的性能的NBT基材料。

Complex perovskite-type (ABO3) ferroelectrics are among the key functional materials due to their ability to convert various types of external force fields into electrical signals. Currently, the most important industrial ferroelectric solid solution is PbZr1-xTixO3 (PZT), exhibiting a composition-driven phase transition, called a morphotropic phase boundary (MPB), at which the dielectric, piezoelectric, pyroelectric, and optoelectric properties are enhanced. Lead is however noxious and hence, environmentally friendly replacements of PZT are necessary to be found. The solid solution (1-x)Na0.5Bi0.5TiO3 -xBaTiO3 (NBT-xBT) is very promising as a Pb-free alternative because, similar to PZT, it has a nearly temperature-independent MPB. However, in spite of the presence of macroscopic polarization, the average structure of NBT-xBT appears to be pseudocubic at MPB, indicating abundant nanoscale structural inhomogeneities and incipient ferroic clustering. Therefore comprehensive in situ structural analyses under different external stimuli (temperature, elastic stress/pressure, electric field) that can boost the ferroic order are required, in order to disclose the intrinsic structural features governing the material properties in alkali-bismuth-based systems. There are a number of studies at different temperatures and electric fields but surprisingly, high-pressure structural analyses of NBT-xBT are scarce and mostly limited to the end members. The current project aims to fill this gap of knowledge. We are going to study the compositional dependence of the pressure-induced structural transformations in NBT-xBT single crystals across the MPB (x = 0, 0.013, 0.048, 0.053, 0.074) at different length scales by Raman spectroscopy, synchrotron X-ray diffuse scattering, and high-precision in-house X-ray diffraction up to ~21 GPa, using the diamond-anvil-cell technique. The comparison between the pressure evolution of the phonons as well as of X-ray diffuse scattering and the equation of state will make it possible to identify the atomistic origin of elastic instabilities and flattening of the local potentials, supporting the property enhancements. The volume compressibility as well as the phonon Grüneisen parameters as a function of composition will be determined. Furthermore, the effect of anisotropic stress versus that of hydrostatic pressure on the local structure and dynamics of NBT-xBT single crystals across the MPB is going to be analyzed on the basis of Raman scattering under non-hydrostatic conditions up to ~5.6 GPa. The expected results should provide a deeper insight into the structure of NBT-xBT and better reveal the intrinsic nanoscale structural difference between Pb-based and Pb-free systems, which in turn can help designing NBT-based materials with properties comparable to those of Pb-rich materials.