Influences of Cation Ratio, Anion Type, and Water Content on Polytypism of Layered Double Hydroxides.

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.