Polytype Transformations in the SO42- Containing Layered Double Hydroxides of Zinc with Aluminum and Chromium: The Metal Hydroxide Layer as a Structural Synthon

Solid-solid inter-polytype transformations are observed during the thermal dehydration of sulfate-containing layered double hydroxides (LDHs). The metal hydroxide layer behaves as a structural synthon and the interconversion of polytypes of rhombohedral and hexagonal symmetries takes place by rigid translations of successive layers by (+/- 1/3, +/- 2/3) relative to one another in the oh plane. These translations are selected among the many possible, as they preserve the coincidence of the symmetry elements of the individual layers and thereby conserve the threefold symmetry of the crystal across the inter-polytype conversions. As a result, these transformations are enthalpically not expensive. These translations are facilitated at near ambient temperatures (30-60 degrees C) by the reversible dehydration of the LDH, which involves the deinsertion/insertion of water molecules within the restricted space of the interlayer region.