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

复合钙钛矿型（ABO3）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PbZr1-xTixO3 (PZT)，表现出成分驱动的相变，称为形态取向相边界（MPB），其介电，压电，热释电和光电性能得到增强。然而，铅是有害的，因此，有必要找到环保的PZT替代品。固溶体（1-x）Na0.5Bi0.5TiO3 -xBaTiO3 （NBT-xBT）作为无铅替代品非常有前途，因为与PZT相似，它具有几乎不受温度影响的MPB。然而，尽管存在宏观极化，但NBT-xBT在MPB下的平均结构表现为赝赝结构，表明存在丰富的纳米级结构不均匀性和早期的铁聚类。因此，需要在不同的外部刺激（温度、弹性应力/压力、电场）下进行全面的原位结构分析，以提高铁有序度，从而揭示控制碱铋基体系中材料性能的内在结构特征。在不同温度和电场条件下对NBT-xBT的高压结构进行了大量的研究，但令人惊讶的是，对NBT-xBT的高压结构分析很少，而且大多局限于末端构件。目前的项目旨在填补这一知识空白。我们将利用金刚石砧细胞技术，利用拉曼光谱、同步加速器x射线漫射散射和高达~21 GPa的高精度内部x射线衍射，研究不同长度尺度下NBT-xBT单晶在MPB （x = 0,0.013, 0.048, 0.053, 0.074）上压力诱导结构转变的成分依赖性。声子的压力演化和x射线扩散散射与状态方程的比较将使我们有可能确定弹性不稳定性和局域势的平坦化的原子起源，支持性质的增强。体积可压缩性以及声子颗粒<s:1>尼森参数作为组成的函数将被确定。此外，在非静水条件下高达~5.6 GPa的拉曼散射的基础上，分析了各向异性应力与静水压力对NBT-xBT单晶在MPB上局部结构和动力学的影响。预期的结果将提供对NBT-xBT结构的更深入的了解，并更好地揭示pb基和无pb体系之间内在的纳米级结构差异，从而有助于设计具有与富pb材料相当性能的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.