Recent diffraction studies on layered double hydroxides have shown that the threelayer 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 p3̅ 12/m or c12/m1) 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.

The imbibition of divalent cations (Zn2+, Ni2+) into half the cation vacancies in Al­(OH)3 results in layered double hydroxides comprising positively charged layers of the composition [M□Al4(OH)12]2+ (□: cation vacancy). Nitrate ions were intercalated for charge neutrality together with water molecules. Structure refinement by the Rietveld method showed that the metal hydroxide crystallized in monoclinic symmetry (space group P121/n1). Upon heating to T = 160 °C, a new phase was obtained which was indexed to a two layer cell of orthorhombic symmetry, suggesting an interpolytype transformation. Using the idealized metal hydroxide layer (layer group p121/a1) as a structural synthon, the different ways of stacking these layers were predicted to arrive at the complete universe of polytypes in this class of compounds. Some of the predicted stacking sequences generated new 2-fold axes, not found in the layer group and resulted in one-layer, two-layer, and three different four-layer polytypes (1O, 2O, and 4O1–3; O: orthorhombic) of orthorhombic symmetry. The high temperature phase can be described as a two-layer polytype (2O) (space group Pn2n) obtained by the rigid translation of the metal hydroxide layers by (1/4, 1/2) relative to one another. Cooling and rehydration restored the as-prepared phase of monoclinic symmetry.

The double hydroxide of Li + and Al 3+ is an anionic clay comprising positively charged metal hydroxide layers and intercalated anions. While the structure of the iono-covalently bonded metal hydroxide layer is well known, relatively less knowledge is available regarding the manner in which the anions and water molecules are packed in the interlayer region. The sulfate ion is of special interest as it can potentially intercalate in a multiplicity of orientations and grow an extended hydration sphere. The sulfate-intercalated double hydroxide was synthesized by the imbibition of Li 2 SO 4 into both the gibbsite and bayerite forms of Al(OH) 3 to obtain layered double hydroxides with the nominal formula Li 2 Al 4 (OH) 12 SO 4 •nH 2 O (n = 4-8). The as-prepared compounds were poorly ordered and did not yield any structural information. Temperatureinduced partial dehydration yielded ordered phases of different structures in the two systems. Simulation of the powder patterns of different model structures, followed by structure refinement in both direct and reciprocal spaces, showed that the gibbsite-derived phase yielded a two-layer polytype of hexagonal symmetry (space group P6 3 /m). The local symmetry of the sulfate ion was close to D 2d with one of the C 2 axes of the SO 4 2− being nearly parallel to the c axis of the crystal. The bayerite-derived phase yielded a one-layer polytype of monoclinic symmetry (space group C2/ m). The sulfate ion was oriented with its C 3 axes tilted away from the stacking direction. Cooling and rehydration (relative humidity~70%) resulted in a reversible expansion of the basal spacing due to the ingress of water molecules from the ambient humidity into the interlayer region. Hydration in both cases resulted in turbostratic disorder. The disorder in the bayerite-derived phase was a result of random intergrowth of motifs with rhombohedral and monoclinic symmetries.

The layered double hydroxides (LDHs) of Li and Al can be synthesized from the four polymorphs of Al(OH) 3 , namely gibbsite, bayerite, nordstrandite, and doyleite. The crystal structure of this class of compounds depends on the type of the precursor used due to their topotactic reaction mechanism. While the LDHs derived from gibbsite and bayerite yield different crystal structures, the incorporation of Li into nordstrandite was expected to yield new LDH structures different from those derived from gibbsite and bayerite. The structure of nordstrandite derived LDHs were however identical to that derived from

Imbibition of lithium sulphate into aluminium hydroxide is known to result in a sulphate-intercalated layered double hydroxide (LDH) of Li and Al. The perchlorate ion has the same size and molecular symmetry as the sulphate ion, but only half its charge. Consequently, twice the number of ClO − 4 ions is needed to balance LDHs the charge on the metal hydroxide layer, compared to the SO 2− 4 ions. In this work, the ClO − 4 -intercalated LDHs were obtained from both the bayerite and gibbsite precursors. Inclusion of the hydration sphere along with the ClO − 4 anion, induced turbostratic disorder in the stacking of the metal hydroxide layers. Temperature-induced dehydration (T ∼ 100-140 • C) brought about a partial ordering in the interlayer region and the ClO − 4 ion oriented itself with one of its C 2 -axes parallel to the metal hydroxide layer. The close packing of ClO − 4 ions could be realized by the complete dehydration of LDH and the distribution of the ClO − 4 ions in all the available interlayer sites. In contrast, within the crystal of the sulphate analogue, the sulphate ions occupy only half the number of interlayer sites. The other half is occupied by the residual water molecules, as the sulphate analogue does not fully dehydrate even at elevated temperatures. This difference in the behaviour of the two LDHs has its origin in the large difference in the hydration enthalpies of the two anions.