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 (CO32-, SO42-, NO3-, 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 NO3- ions exhibit a polytype transition from 3 R1 (three-layer rhombohedral polytype) to 1 T (one-layer trigonal polytype) with increasing water content. NO3- ions exhibit a D3 h point group symmetry at low water contents. The polytype transition coincides with the complete transformation into tilted NO3- ion with a C2 v point group symmetry. The transition appears at a lower water content when the Mg/Al ratio is lower. LDHs with SO42- ions exhibit a three-stage polytypism. The first and last stages are 3 R1. 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 SO42- ions with a C s point group symmetry is characteristic for the intermediate stage, while mostly SO42- ions exhibit a C3 v symmetry. There is no clear relevance between cation ratio and water content at which a polytype transition happens. The configurational adjustments of NO3- and SO42- ions facilitate the swelling behavior of LDHs. LDHs with CO32- or Cl- ions always maintain a 3 R1 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.