High-Pressure Behavior of Framework Structures

Rossmann-0105864框架结构包括在地球科学、固态化学、凝聚态物理学和材料科学中具有重要意义的各种天然和合成化合物。骨架结构是这样的材料，其原子结构可以用紧密键合的原子团如SiO 4或PO 4连接在一起来描述，这些原子团可以被认为形成相对刚性的多面体，多面体的角由阴离子如氧形成。然后通过这些多面体的缩合，即通过共享氧阴离子，形成框架。 由四面体基团如SiO 4和AlO 4形成的骨架结构的实例包括主要的矿物组，如长石（地壳的60%）和沸石，它们是化学工业中最重要的催化剂家族。其他框架，包括在地球深部稳定的矿物，如石榴石，包含形成网络的四面体和八面体。钙钛矿，由于其电学和弹性性质，可能是最重要的一组技术化合物，是完全由角连接八面体组成的框架的例子。了解这些常见结构类型的行为原理非常重要，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。对于矿物物理学来说，人们需要预测这些次要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种钙钛矿的行为也很重要，例如在许多工业设备中使用的铁磁弛豫体和铁磁弹性体。高压研究可以为框架的一般性质提供重要的见解，原因有几个。首先，压力是一种比温度更强的驱动力，因此人们可以通过压缩结构而不是加热它来探索更大范围的结构行为;其次，压力允许直接探索原子间相互作用势的排斥机制。第三，确定材料的晶胞常数不仅可以得到作为压力函数的体积（用于热力学计算和岩石学）和对地球物理解释很重要的体积模量，而且还可以得到各个弹性模量的某些组合。我们建议进行的研究的理念是通过单晶X射线衍射研究压力下选定的结构类型的演变，压力高达10 GPa。目前的方法不允许在高压下足够精确地确定晶体的结构参数，以识别在施加压力时骨架结构中发生的所有小变化。因此，这项拨款中提出的工作的很大一部分是致力于改进实验技术，我们预计这将有更广泛的应用，而不仅仅是高压研究。从具体的结果中，我们期望能够得出有关框架结构在一系列条件下的行为的一般原则。

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.