Polytypism in Alumite-like Layered Double Hydroxides of M (Zn2+, Ni2+) and Al: A Structural Transformation from Monoclinic to Orthorhombic Symmetry

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 square Al-4(OH)(12)](2+) (square: 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 P12(1)/n1). Upon heating to T = 160 degrees 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 p12(1)/n1) 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 4O(1-3); 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.