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

Abstract

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<i>R</i>₁ (three-layer rhombohedral polytype) to 1<i>T</i> (one-layer trigonal polytype) with increasing water content. NO₃^– ions exhibit a <i>D</i>_3<i>h</i> point group symmetry at low water contents. The polytype transition coincides with the complete transformation into tilted NO₃^– ion with a <i>C</i>_2<i>v</i> 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<i>R</i>₁. The intermediate stage could be 1<i>T</i> or a mixture of different <i>O</i>(octahedra)-type interlayers, which depends on the cation ratio. The relative popularity of SO₄^2– ions with a <i>C</i>_<i>s</i> point group symmetry is characteristic for the intermediate stage, while mostly SO₄^2– ions exhibit a <i>C</i>_3<i>v</i> 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<i>R</i>₁ 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