Structural Cohesion of M^II-M^III Layered Double Hydroxides Crystals: Electrostatic Forces and Cationic Polarizing Power

Abstract

Layered double hydroxides (LDHs), [MII(1–x)MIIIx(OH)2]x+[An–]x/n·zH2O], with various cations and diverse stoichiometries were synthesized by three different coprecipitation routes to highlight the effect of the synthesis method on the composition range of the corresponding materials. While morphology and crystallinity are strongly dependent on the coprecipitation routes, the composition range remains unchanged. Thus, the nature of the cations present in the layer sheet of the material plays an important role in its stability. Further insight into the structural stability of the materials was achieved by two electrostatic models. By computing attractive and repulsive forces involving cationic and anionic punctual charges, it is shown that the most favorable system corresponds to a layer charge of 0.17, slightly lower than naturally occurring minerals for which the layer charge is generally 0.25. This difference may be explained by weaker forces such as hydrogen bonding or crystal cohesion. Thus, a second model is proposed based on the size and the electronic configuration of the cations, that is, the polarizing power, and the effect of the cationic polarizing power on the intensity of the hydrogen bond is described. This model is successful in predicting the lower limit of composition of LDH materials, as a function of the nature of the cations present in the layer sheet.