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

这个提案是A.M.教授和Glazer教授（牛津克拉伦登实验室晶体学组）和P.A. Thomas教授和M.E.Smith教授（华威大学物理系）。总体目标是首次在铌酸锂和钽酸锂系列（以下简称LNT）固溶体的绝对晶体结构与它们的一些非常不寻常的物理性质之间建立关键联系。在这个提议中，一个特别的焦点和关键的兴趣点是，在该系列中存在一种在室温下双折射为零的组合物，因此晶体成为光学各向同性，但仍保持电极性，这是非线性光学功能铁电材料中独特而极其奇怪的性质组合。作为我们研究计划的一部分，我们打算密切研究这种迷人的成分，以确定这种材料是否有潜力作为光学传感应用中异常敏感的组件，这具有很高的技术重要性和及时的相关性。对于其他LNT组分，可以通过提高温度获得光学各向同性点，从而在二维组分-温度图中存在零双折射点轨迹线。我们的目标是从基本角度了解LNT系列中这种行为的发生，同时牢记基于成分和温度调谐组合的设备的潜力。在一个全新的和创新的转折中，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射线衍射和高压下固态核磁共振的强大方法组合，我们将在三维参数空间中绘制零双折射轮廓的出现情况，以构建成分-温度-压力（x-T-P）图。由于LN本身具有较大的光弹性和压电系数，我们预计零双折射点的压力依赖性非常高，从而为高灵敏度和可调压力传感器开辟了潜力。我们的研究将集中在专家x射线结构分析上，包括绝对极性测定（即利用异常x射线散射确定结构中偏离中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高线提供必要的数据。除此之外，双折射成像显微镜将用于绘制LNT成分的光学特性，从而绘制出温度，压力和光学波长的零双折射轮廓。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畸变和阳离子位移在组成无序晶体（如LNT家族）的结构-性质关系中所起的作用。这些将扩展到高温，使用为93Nb核磁共振构建的专用探针，最终，随着高压核磁共振的上线，敏感的零双折射成分的压力高达5 GPa。总之，这项研究方案结合了最先进的方法，对LNT系列进行基础性的新科学研究。它将揭示新的材料物理学，并回答一些长期存在的问题在x-T-P空间的LNT。最终，最具推测性的是，它可能会在未来几年为基于这种最不寻常的物理性质组合的设备的开发提供新的动力。

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.