Interdisciplinary Graduate Program in Biomolecular Science & Engineering, Institute for Collaborative Biotechnologies, Materials Research Laboratory, University of California, Santa Barbara, CA 93106-5160, USA.
Chemistry. 2010 Sep 3;16(33):9998-10006. doi: 10.1002/chem.201000661.
Structures of layered metal hydroxides are not well described by traditional crystallography. Total scattering from a synthesis-controlled subset of these materials, as described here, reveals that different cobalt coordination polyhedra cluster within each layer on short length scales, offering new insights and approaches for understanding the properties of these and related layered materials. Structures related to that of brucite [Mg(OH)(2)] are ubiquitous in the mineral world and offer a variety of useful functions ranging from catalysis and ion-exchange to sequestration and energy transduction, including applications in batteries. However, it has been difficult to resolve the atomic structure of these layered compounds because interlayer disorder disrupts the long-range periodicity necessary for diffraction-based structure determination. For this reason, traditional unit-cell-based descriptions have remained inaccurate. Here we apply, for the first time to such layered hydroxides, synchrotron X-ray total scattering methods-analyzing both the Bragg and diffuse components-to resolve the intralayer structure of three different alpha-cobalt hydroxides, revealing the nature and distribution of metal site coordination. The different compounds with incorporated chloride ions have been prepared with kinetic control of hydrolysis to yield different ratios of octahedrally and tetrahedrally coordinated cobalt ions within the layers, as confirmed by total scattering. Real-space analyses indicate local clustering of polyhedra within the layers, manifested in the weighted average of different ordered phases with fixed fractions of tetrahedrally coordinated cobalt sites. These results, hidden from an averaged unit-cell description, reveal new structural characteristics that are essential to understanding the origin of fundamental material properties such as color, anion exchange capacity, and magnetic behavior. Our results also provide further insights into the detailed mechanisms of aqueous hydrolysis chemistry of hydrated metal salts. We emphasize the power of the methods used here for establishing structure-property correlations in functional materials with related layered structures.
层状金属氢氧化物的结构不能用传统晶体学很好地描述。本文所述的对这些材料的一部分进行合成控制的全散射研究表明，不同的钴配位多面体在短程尺度内聚集在每个层内，为理解这些和相关的层状材料的性质提供了新的见解和方法。与水镁石[Mg(OH)(2)]结构相关的结构在矿物世界中无处不在，提供了从催化和离子交换到螯合和能量转换等多种有用功能，包括在电池中的应用。然而，由于层间无序破坏了基于衍射的结构确定所需的长程周期性，这些层状化合物的原子结构一直难以解析。出于这个原因，传统的基于晶胞的描述仍然不准确。在这里，我们首次将同步加速器 X 射线全散射方法应用于这些层状氢氧化物，分析布拉格和漫散射成分，以解析三种不同的α-钴氢氧化物的层内结构，揭示金属位点配位的性质和分布。通过全散射证实，具有不同比例的八面体和四面体配位钴离子的不同含氯离子化合物是通过水解动力学控制制备的。实空间分析表明，在层内多面体的局部聚集，表现为具有固定分数的四面体配位钴位的不同有序相的加权平均值。这些从平均晶胞描述中隐藏的结果揭示了新的结构特征，对于理解基本材料性质（如颜色、阴离子交换能力和磁行为）的起源至关重要。我们的结果还为水合金属盐的水解释化学的详细机制提供了进一步的见解。我们强调这里使用的方法在具有相关层状结构的功能材料中建立结构-性质相关性的强大功能。
