Insights on the structural chemistry of hydrocalumite and hydrotalcite-like materials:: Investigation of the series Ca2M3+(OH)6Cl • 2H2O (M3+:Al3+, Ga3+, Fe3+, and Sc3+) by X-ray powder diffraction

Hydrotalcite and hydrocalumite are two close minerals belonging to the layered double hydroxide family. Both structures are based on positive brucite-like layers alternating with layers containing anions and water molecules. Most of synthetic (LDHs) are hydrotalcite-like materials. On the other hand, the hydrocalumite structure type is rare for those less broad in composition, typically Ca2+ and Al3+ in the hydroxide layers. In order to get further insight into the conditions of stabilization of this structure type, we have undertaken the synthesis and the structural characterization by powder X-ray diffraction of the series Ca2M3+ (OH)(6)Cl.2H(2)O(M3+:Al3+, Ga3+, Fe3+ and Sc3+). The incorporation of Sc3+ ions is quite original. All phases crystallize in the rhombohedral space group R-3 resulting in a three-layers polytype. The main consequence of the replacement of Al3+ cations by large M3+ cations is a compression of the octahedral layers like it proceeds in hydrotalcite-like materials. The existence of Sc-containing phase allows us to say that it is the size of Ca2+ ions and the pronounced anisotropy of coordination spheres around Ca2+ and M-3+ which are responsible for the ordered distribution of cations in hydrocalumite-like materials. (C) 2002 Elsevier Science (USA).