High-Pressure Behavior of Framework Structures

框架结构涵盖了在地球科学、固体化学、凝聚态物理和材料科学中重要的广泛的天然和合成化合物。框架结构是这样一种材料，其原子结构可以用紧密结合的原子群（如SiO4或PO4）连接在一起来描述，它们可以被认为形成相对刚性的多面体，其角是由阴离子（如氧）形成的。然后通过这些多面体的缩合形成框架，即通过氧阴离子的共享。由四面体基团（如SiO4和AlO4）形成的框架结构的例子包括主要的矿物群，如长石（占地壳的60%）和沸石（化学工业中最重要的催化剂家族）。其他框架，包括在地球深处稳定的矿物，如石榴石，包含形成网络的四面体和八面体。钙钛矿可能是技术上最重要的一类化合物，因为它具有电性和弹性，是完全由角连接的八面体组成的框架的例子。理解这些常见结构类型的行为原理很重要，原因有很多。例如，在地球化学中，我们需要能够预测其他组分固溶成端元化合物的极限。对地球物理学来说，人们需要预测这些微小的化学成分对矿物弹性的影响。弹性和结构性质的相互作用对于理解这些钙钛矿的行为也很重要，例如在许多工业设备中使用的铁弛豫剂和铁弹性剂。高压研究可以为框架的一般特性提供重要的见解，原因有几个。首先，压力在能量上是比温度强大得多的驱动力，因此人们可以通过压缩结构而不是加热来探索更大范围的结构行为。第二，压力允许直接探索原子间势的排斥状态。第三，确定材料的单胞常数不仅可以得到体积作为压力的函数（用于热力学计算和岩石学）和体积模量（这对地球物理解释很重要），还可以得到个别弹性模量的某些组合。我们提出的研究理念是通过单晶x射线衍射研究在压力达到10 GPa的压力下选定结构类型的演变。目前的方法不允许足够精确地确定高压下晶体的结构参数，以确定在施加压力时框架结构中发生的所有微小变化。因此，在这项拨款中提出的工作的重要部分是致力于改进实验技术，我们预计这将比单独的高压研究有更广泛的应用。从具体的结果中，我们期望能够推导出关于框架结构在一系列条件下的行为的一般原理。

RossEAR-0105864Framework structures encompass a wide range of natural and synthetic compounds of importance in the Earth sciences, solid state chemistry, condensed matter physics, and materials sciences. Framework structures are materials whose atomic structure can be described in terms of the linking together of tightly-bonded groups of atoms such as SiO4 or PO4 that can be thought of forming relatively rigid polyhedra whose corners are formed by the anions such as oxygen. The frameworks are then formed by the condensation of these polyhedra, i.e. by sharing of the oxygen anions. Examples of framework structures formed of tetrahedral groups such as SiO4 and AlO4 include major groups of minerals such as the feldspars (60% of the Earths crust), and zeolites that are the most important family of catalysts in the chemical industry. Other frameworks, including minerals stable in the deeper Earth such as garnets, incorporate network-forming tetrahedra and octahedra. Perovskites, which are probably the single most technologically-important group of compounds because of their electrical and elastic properties, are examples of frameworks composed entirely of corner-linked octahedra. Understanding the principles of behavior of these common structure types is important for a number of reasons. For example, in geochemistry we need to be able to predict the limit of solid solution of other components into end-member compounds. For geophysics one needs to predict the influence of these minor chemical components on the elasticity of minerals. The interaction of elastic and structural properties is also important for understanding the behavior of such perovskites such as ferro-relaxors and ferro-elastics that are used in many industrial devices. High-pressure studies can provide important insights into the general properties of frameworks for several reasons. First, pressure is energetically a far stronger driving force than temperature and thus one can explore a far greater range in structural behavior by compressing a structure rather than heating it. Second, pressure allows the repulsive regime of the inter-atomic potential to be explored directly. And, third, determination of the unit-cell constants of a material yield not only the volume as a function of pressure (for thermodynamic calculations and petrology) and the bulk modulus which is important for geophysical interpretations, but also certain combinations of individual elastic moduli. The philosophy of the research that we propose to undertake is to study the evolution of selected structure types under pressure by single-crystal X-ray diffraction up to pressures of 10 GPa. Current methodologies do not allow the sufficiently precise determination of the structural parameters of crystals under high pressures to identify all of the small changes that occur in framework structures as pressure is applied. Therefore a significant part of the work proposed in this grant is devoted to the improvement in the experimental techniques, which we expect will have wider applications than to high-pressure research alone. From the specific results we expect to be able to derive general principles concerning the behavior of framework structures under a range of conditions.