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 (CO₃^2-, SO₄^2-, 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 D_{3 h} point group symmetry at low water contents. The polytype transition coincides with the complete transformation into tilted NO₃^- ion with a C_{2 v} point group symmetry. The transition appears at a lower water content when the Mg/Al ratio is lower. LDHs with SO₄^2- 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 SO₄^2- ions with a C _s point group symmetry is characteristic for the intermediate stage, while mostly SO₄^2- ions exhibit a C_{3 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 SO₄^2- ions facilitate the swelling behavior of LDHs. LDHs with CO₃^2- 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.