CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry , Chinese Academy of Sciences (CAS) , Guangzhou 510640 , China.
University of Chinese Academy of Sciences , 19 Yuquan Road , Beijing 100049 , China.
Inorg Chem. 2018 Jun 18;57(12):7299-7313. doi: 10.1021/acs.inorgchem.8b00949. Epub 2018 Jun 4.
Layered double hydroxides (LDHs) are a significant sink of anions (CO, SO, NO, Cl, etc.) and divalent transition-metal cations in soil. The anion exchange capacity gives rise to functional materials. The stability of LDHs is determined by the interaction between cation-bearing layers and intercalated water and anions, which is correlated with polytypism and coordination structure. A systematic investigation is performed to show the influence of cation ratio, anion type, and water content on polytypism, swelling behavior, and interlayer structure of Mg-Al-LDHs using molecular dynamics simulations. LDHs intercalated with NO ions exhibit a polytype transition from 3 R (three-layer rhombohedral polytype) to 1 T (one-layer trigonal polytype) with increasing water content. NO ions exhibit a D point group symmetry at low water contents. The polytype transition coincides with the complete transformation into tilted NO ion with a C point group symmetry. The transition appears at a lower water content when the Mg/Al ratio is lower. LDHs with SO ions exhibit a three-stage polytypism. The first and last stages are 3 R. The intermediate stage could be 1 T or a mixture of different O(octahedra)-type interlayers, which depends on the cation ratio. The relative popularity of SO ions with a C point group symmetry is characteristic for the intermediate stage, while mostly SO ions exhibit a C symmetry. There is no clear relevance between cation ratio and water content at which a polytype transition happens. The configurational adjustments of NO and SO ions facilitate the swelling behavior of LDHs. LDHs with CO or Cl ions always maintain a 3 R polytype irrespective of water content and hardly swell. The configurations of anions and water reflect local coordination structure due to hydrogen bonds. The layer-stacking way influences long-ranged Coulombic interactions. Hydrogen-bonding structure and long-ranged Coulombic interactions collectively determine polytypism and stability of LDHs.
层状双氢氧化物 (LDHs) 是土壤中阴离子（CO、SO、NO、Cl 等）和二价过渡金属阳离子的重要汇。阴离子交换容量赋予了功能材料的特性。LDHs 的稳定性取决于带阳离子的层与插层水和阴离子之间的相互作用，这与多型性和配位结构有关。通过分子动力学模拟，对阳离子比、阴离子类型和含水量对 Mg-Al-LDHs 的多型性、溶胀行为和层间结构的影响进行了系统的研究。随着含水量的增加，NO 离子插层的 LDHs 从 3R（三层菱面体多型性）转变为 1T（一层三角多型性）。在低含水量时，NO 离子表现出 D 点群对称性。这种多型性转变与完全转变为具有 C 点群对称性的倾斜 NO 离子相吻合。当 Mg/Al 比较低时，转变出现在较低的含水量下。具有 SO 离子的 LDHs 表现出三阶段的多型性。第一和最后阶段是 3R。中间阶段可能是 1T 或不同 O(八面体)型层间的混合物，这取决于阳离子比。具有 C 点群对称性的 SO 离子的相对流行是中间阶段的特征，而大多数 SO 离子表现出 C 对称性。多型性转变发生时，阳离子比和含水量之间没有明显的相关性。NO 和 SO 离子的构型调整促进了 LDHs 的溶胀行为。无论含水量如何，具有 CO 或 Cl 离子的 LDHs 始终保持 3R 多型性，几乎不溶胀。阴离子和水的构型由于氢键的存在反映了局部配位结构。层堆叠方式影响长程库仑相互作用。氢键结构和长程库仑相互作用共同决定了 LDHs 的多型性和稳定性。
