Reappraisal of Polytypism in Layered Double Hydroxides: Consequences of Cation Ordering in the Metal Hydroxide Layer

Recent diffraction studies on layered double hydroxides have shown that the three-layer polytypes that were thought to crystallize in rhombohedral symmetry are actually one-layer polytypes of monoclinic symmetry. However, the prevailing Bookin and Drits (1993) scheme of polytypism, which is based on the widely accepted cation-disordered structure model, fails to predict the occurrence of low symmetry (monoclinic and orthorhombic) polytypes among the layered double hydroxides. In this work, a cation-ordered metal hydroxide layer (layer group <i>p</i>3̅12/<i>m</i> or <i>c</i>12/<i>m</i>1) is chosen as the basic building block. Application of the structural synthon approach enables the description of the complete universe of polytypes comprising 1H, 1M_1–7, 2H, 2O, 3R, 3H, 6H, and 6R among others (M: monoclinic; O: orthorhombic). These polytypes are characterized by their unique stacking vectors. The polytypes of large unit cell volume are obtained by a combination of two or more stacking vectors. This work has relevance to the understanding of several mineral structures, especially those with large unit cells