Fletcher Rachel E, Wells Stephen A, Leung Ka Ming, Edwards Peter P, Sartbaeva Asel
Department of Chemistry, University of Bath, Bath BA2 7AY, England.
Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, England.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2015 Dec 1;71(Pt 6):641-7. doi: 10.1107/S2052520615018739.
Framework materials have structures containing strongly bonded polyhedral groups of atoms connected through their vertices. Typically the energy cost for variations of the inter-polyhedral geometry is much less than the cost of distortions of the polyhedra themselves - as in the case of silicates, where the geometry of the SiO4 tetrahedral group is much more strongly constrained than the Si-O-Si bridging angle. As a result, framework materials frequently display intrinsic flexibility, and their dynamic and static properties are strongly influenced by low-energy collective motions of the polyhedra. Insight into these motions can be obtained in reciprocal space through the rigid unit mode' (RUM) model, and in real-space through template-based geometric simulations. We briefly review the framework flexibility phenomena in energy-relevant materials, including ionic conductors, perovskites and zeolites. In particular we examine the flexibility window' phenomenon in zeolites and present novel results on the flexibility window of the EMT framework, which shed light on the role of structure-directing agents. Our key finding is that the crown ether, despite its steric bulk, does not limit the geometric flexibility of the framework.
骨架材料具有这样的结构，其中包含通过顶点相连的强键合多面体原子团。通常，多面体间几何形状变化的能量成本远低于多面体自身畸变的成本——就像硅酸盐的情况一样，其中SiO4四面体基团的几何形状比Si - O - Si桥连角受到的约束要强得多。因此，骨架材料常常表现出固有的柔韧性，其动态和静态性质受到多面体低能量集体运动的强烈影响。通过“刚性单元模式”（RUM）模型可以在倒易空间中获得对这些运动的洞察，而通过基于模板的几何模拟可以在实空间中获得。我们简要回顾了与能量相关材料中的骨架柔韧性现象，包括离子导体、钙钛矿和沸石。特别地，我们研究了沸石中的“柔韧性窗口”现象，并给出了关于EMT骨架柔韧性窗口的新结果，这些结果揭示了结构导向剂的作用。我们的关键发现是，冠醚尽管具有较大的空间体积，但并不限制骨架的几何柔韧性。
