A Crystal Chemical Approach to a Cation-Ordered Structure Model for Carbonate-Intercalated Layered Double Hydroxides

Layered double hydroxides comprise a stacking of positively charged metal hydroxide layers with anions and water molecules included in the interlayer galleries. Among the anions, the carbonate ion is the most ubiquitous in both mineral and laboratory synthesized phases. Taylor (1973) suggested that the carbonate ion (molecular symmetry D_3h) prefers a trigonal prismatic interlayer site (local symmetry D_3h), whereby the hydrogen bonding with the metal hydroxide layer is maximized. However, the cation ordered structure models of hexagonal symmetry include interlayer sites which are exclusively trigonal antiprisms (local symmetry D_3d). In keeping with Taylor's criterion, a hexagonal stacking of metal hydroxide layers does not permit the inclusion of carbonate ions in the interlayer. In this work, a crystal chemical approach is adopted based on the translationgleiche subgroups of hexagonal and cubic summits to arrive at a structure model based on the space group C2/m. In this structure, not only is the 3-fold symmetry of metal coordination retained, but also interlayer sites of ∼D_3h symmetry are generated to host the intercalated carbonate ions. Using this model, the structures of a cohort of carbonate-intercalated layered double hydroxides are refined.