Crystallography and Properties of Lithium Niobate-Tantalate Solid Solutions:towards novel optically isotropic, electrically polar materials

这项建议是牛津大学克拉伦登实验室结晶学小组的A.M.Glazer教授和华威大学物理系的P.A.Thomas和M.E.Smith教授合作的结果。总的目标是首次在Nb酸锂和LiTa(以下简称LNT)系列固溶体的绝对晶体结构和它们的一些非常不寻常的物理性质之间建立关键联系。这一提议中的一个特别的焦点和关键感兴趣的点是，在室温下双折射为零的系列中存在组合物，使得晶体变得光学各向同性，但仍保持电极性，这是非线性光学功能铁电材料中独特且极其奇怪的属性组合。作为我们研究方案的一部分，我们打算仔细研究这种迷人的成分，以确定这种材料是否有潜力在具有高度技术重要性和及时相关性的光学传感应用中作为异常敏感的部件。对于其他LNT组分，通过升高温度可以得到光学各向同性点，从而在二维组成-温度图中存在一条零双折射点的轨迹线。我们的目标是从根本上理解LNT系列中这种行为的发生，同时牢记基于成分和温度调节组合的设备的潜力。在一个全新和创新的转折中，我们将首次研究附加参数压力对LNT结构和性质的影响，特别是在零双折射点附近。利用双折射成像显微镜、X射线衍射仪和高压固体核磁共振的强大组合，我们将在三维参数空间中绘制出零双折射的轮廓线，以构建组成-温度-压力(x-T-P)图。由于LN本身具有很大的光弹性系数和压电系数，我们预计零双折射点的压力依赖性将非常高，从而为开发高灵敏度和可调谐的压力传感器提供了可能性。我们的研究将集中于专家X射线结构分析，包括使用反常X射线散射确定绝对极性(即确定结构中偏心离子的方向与电极化感之间的关系)。这些研究将扩展到非环境温度和压力，以便填写参数图，并为解释零双折射等值线提供必要的数据。此外，双折射成像显微镜将被用来绘制LNT组分的光学性质以及随温度、压力和光学波长变化的零双折射等高线。多核固体核磁共振将包括7Li，17O和93Nb，特别是为了了解八面体扭曲和阳离子位移在成分无序晶体(如LNT家族)的结构-性质关系中所起的作用。使用为93Nb核磁共振构建的专用探头，这些测试将扩展到高温，并最终随着高压核磁共振的上线，扩展到高达5 Gpa的压力，用于敏感的零双折射成分。总而言之，这一研究方案结合了最先进的方法，对LNT系列进行了基础性质的新颖科学研究。它既将揭示新材料物理，又将回答LNT在x-T-P空间中的一些长期存在的问题。最终，也是最有可能的是，它可能会在未来几年为基于这种最不寻常的物理特性组合的设备的发展提供新的动力。

This proposal is a collaboration between Professor A.M. Glazer (Crystallography Group, Clarendon Laboratory, Oxford) and Professors P.A. Thomas and M.E.Smith (Physics Dept., University of Warwick). The overall aim is to make for the first time, the crucial link between the absolute crystal structure of solid solutions in the lithium niobate and lithium tantalate series (hereafter LNT) and some of their highly unusual physical properties. A particular focus and point of key interest in this proposal is the existence of a composition in the series where the birefringence is zero at room temperature, so that the crystals become optically isotropic and yet remain electrically polar, which is an unique and extremely odd combination of properties in a nonlinear-optical, functional ferroelectric material. As part of our research programme, we intend to investigate this fascinating composition closely with a view to establishing whether such a material has potential as an unusually sensitive component in optically-based sensing applications, which are of high technical importance and timely relevance. For other LNT compositions, the point of optical isotropy can be obtained by raising the temperature, so that a locus line of zero birefringence points exists in the two-dimensional composition-temperature map. It is our goal to understand the occurrence of this behaviour across the LNT series from a fundamental point of view, whilst keeping in mind the potential for devices based on a combination of compositional and temperature tuning. In an entirely new and innovative twist, we will investigate for the first time the effect of the additional parameter pressure on the structure and properties of LNT in general, and particularly in the vicinity of the points of zero birefringence . Using birefringent imaging microsocopy, x-ray diffraction and solid state NMR at elevated pressures in a powerful combination of methodologies, we will map out the occurrence of the contours of zero birefringence in a three-dimensional parameter space to construct a composition-temperature-pressure (x-T-P) diagram. Since LN itself has large photoelastic and piezoelectric coefficients, we expect the pressure-dependence of the zero-birefringence points to be extremely high, thereby opening up the potential for a highly-sensitive and tunable pressure sensor. Our research will concentrate on expert x-ray structural analysis including absolute polarity determination (that is determination of the relationship between the direction of off-centre ions in the structures and the sense of electrical polarization) using anomalous x-ray scattering. These studies will be extended to non-ambient temperatures and pressures in order to fill out the parameter map and give the necessary data for interpretation of the zero birefringence contours. Alongside this, birefringent imaging microscopy will be used to map out the optical properties and thus, the zero birefringence contours of LNT compositions as a function of temperature, pressure and optical wavelength. Multinuclear solid state NMR will include 7Li, 17O and 93Nb, particularly to understand the role that octahedral distortions and cation displacements play in structure-property relations for compositionally-disordered crystals such as the LNT family. These will be extended to high temperatures using a dedicated probe constructed for 93Nb NMR and ultimately, to pressures of up to 5 GPa for sensitive zero-birefringence compositions as high-pressure NMR comes on-line. In summary, this research programme combines state-of-the-art methodologies to undertake novel science of a fundamental nature on the LNT series. It will both reveal new materials physics and answer some long-standing questions in the x-T-P space for LNT. Ultimately, and most speculatively, it may provide a new impetus for the development of devices based on this most unusual combination of physical properties in future years.