Solid-State NMR Characterization of the Local Structure of Lead-Free Relaxor Ferroelectrics

钛酸钠铋及其与钡（100-x）（Na1/2,Bi1/2）TiO3 (x)BaTiO3 （NBT xBT）的固溶体作为铅基压电陶瓷的一种有前途的环保替代品而脱颖而出。这些本质上不均匀的材料表现出小幅度和短相干长度的结构畸变，这对仅对平均结构敏感的传统方法的表征提出了挑战。因此，这些材料中结构-性能关系的关键方面仍然不清楚，例如化学修饰的作用，相变的性质（温度或电场诱导）以及弛豫或铁电态的发生。在这个项目中，我们采用固态核磁共振（ssNMR）光谱来面对这类材料带来的挑战，并在局部尺度上了解它们的结构。这将通过分析静态的、一维的MAS，特别是二维的3QMAS核磁共振波谱来实现，它可以根据23Na和其他核磁共振活性核的核相互作用来描述局部结构。对相关核磁共振参数温度依赖性的研究，将有助于揭示NBT-xBT组分的局部结构在退极化温度（Td）低于铁电弛豫转变温度（TF R）和介电介电常数最高温度（Tm）及以上的整个温度范围内发生的变化，包括未极化和极化。局部结构的动力学方面将通过研究核磁共振弛豫时间来研究铁电态和弛豫态，旨在确定这些材料中存在的局部结构波动的性质。第一个应用周期的重点是用固体核磁共振表征（Na1/2,Bi1/2）TiO3固溶体与钛酸钡（NBT xBT, 0≤x≤15）的局部结构和电极化的依赖关系。在第二个申请阶段，我们首先希望(a)完成对NBT - xBT体系中明显立方相性质的研究。为此，我们将(a)解决MAS-NMR谱中缺失的边带强度问题，(b)完成与场相关的23na -T1弛豫曲线，以便能够有足够的数据来模拟T1-和t2 -色散，并揭示有关表观立方相的动力学性质的信息。同时，我们希望(b)利用这些技术来详细分析BaTiO3-CaTiO3（钛酸钡钙，BCT）的固溶体，其极化/去极化行为，沉淀硬化过程及其对这些材料内部极化/去极化过程的影响。我们对这类材料的兴趣是由它们有利的电损耗特性引起的。

Sodium-bismuth titanate and its solid-solutions with barium (100-x)(Na1/2,Bi1/2)TiO3 (x)BaTiO3 (NBT xBT) stand out as a promising and environmentally friendly alternative to lead-based piezoelectric ceramics. These intrinsically inhomogeneous materials exhibit structural distortions with small magnitude and short coherence length, which pose a challenge to their characterization with conventional methods that are sensitive only to the average structure. As a consequence, key aspects of the structure-property relations in these materials are still unclear, as for example the role of chemical modifications, the nature of phase transitions (either temperature or electric field-induced) and the occurrence of a relaxor ferroelectric state. In this project, we employ solid-state nuclear magnetic resonance (ssNMR) spectroscopy in order to face the challenges posed by this class of materials and to learn about their structure at the local scale. This will be achieved by the analysis of static, one-dimensional MAS and in particular two-dimensional 3QMAS NMR spectra, which enable a description of the local structure in terms of the nuclear interactions present for 23Na and other NMR-active nuclei available. The investigation of the temperature dependence of relevant NMR parameters should shed light on the changes occurring to the local structure of the NBT-xBT compositions, both unpoled and poled, throughout the temperature range from below the depolarization temperature (Td) over the ferroelectric-to-relaxor transition temperature (TF R) and the dielectric permittivity’s maximum temperature (Tm) and above. Dynamical aspects of the local structure will be investigated both in the ferroelectric and relaxor states by the investigation of NMR relaxation times, aiming at the identification of the nature of local structural fluctuations present in these materials. The focus of the first application period was the characterization of the dependence of the local structure and electric polarization of (Na1/2,Bi1/2)TiO3 solid-solutions with barium titanate (NBT xBT, 0≤x≤15) with solid-state NMR. In the second application period we first want to (a) finish our studies on the nature of the apparently cubic phase in the NBT xBT system. For this we will (a) solve the problem of the missing side-band intensity in the MAS-NMR spectra and (b) complete the field dependent 23Na-T1-relaxation curves in order to be able to have enough data to model the T1- and T2-dispersion and reveal information about the dynamical nature of the apparent cubic phase. In parallel, we want (b) to employ these techniques to a detailed analysis of solid solutions of BaTiO3-CaTiO3, (barium calcium titanate, BCT), its poling/depoling behavior, the precipitation hardening process and its effect on the poling/depoling process inside these materials. Our interest in this type of materials is triggered by their favorable electric loss properties.