Polytype Transformations in the SO₄^2– Containing Layered Double Hydroxides of Zinc with Aluminum and Chromium: The Metal Hydroxide Layer as a Structural Synthon

Abstract

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 ab 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 °C) by the reversible dehydration of the LDH, which involves the deinsertion/insertion of water molecules within the restricted space of the interlayer region.

Structural Synthon Approach to Predict the Possible Polytypes of Layered Double Hydroxides

“…In an earlier paper, [14] we described a 'structural synthon' as a certain packing of atoms, which like its better known mo- www.zaac.wiley-vch.de lecular analogue, the 'supramolecular synthon', [20] repeats itself in a very diverse range of solids. The AC layer with the generalized composition [MX 2 ] is found in a very diverse range of materials such as metal sulfides MS 2 (M = Ti, Mo, Nb, Ta), [21,22] metal selenides MSe 2 (M = Mo, Ta), [23] divalent halides MX 2 (M = Ca, Mg, Fe, Co, Ni, Cd), [24,25] metal hydroxides M(OH) 2 (M = Mg, Ni, Co), [26,27] birnessite-type oxides MO 2 (M = Mn), [28,29] lithium oxides LiMO 2 [30,31] and basic salts M(OH) 2-x A x (M = Ni, Co; x = 0.66-0.25; A = Cl, NO 3 ).…”

“…[13] Sulfate-LDHs exhibit interpolytype transformations (1H↔3R) at low temperatures indicating that the energy barrier between two polytypes for sulfate LDHs is low. [14] In contrast with the carbonate ions, nitrate ions are known to coordinate in the C 2v mode, in the interlayer of LDHs with x = 0.33 composition. [19] In this mode of coordination, one of the NO bonds is parallel to the c-crystallographic axis.…”

“…[12] Laboratory-synthesized sulfate LDHs crystallize in the structure of 1H and 3R 1 polytypes depending on the precipitation conditions and exhibit solid-solid interpolytype transformations as a function of temperature and relative humidity. [13,14] In many instances, laboratory-synthesized LDHs crystallize with the incorporation of a very large number of stacking faults, which can indeed be interpreted in terms of the intergrowths of more than one polytypes. [15] Thus, the Mg/Al, Co/Fe and Ni/Al LDHs have been shown to be random intergrowths of the 3R 1 and 2H 1 polytypes.…”

DFT Study of Polymorphism in Al(OH)3: A Structural Synthon Approach

“…Within the specific and limited context of polytypism in layered materials, this difficulty was sought to be mitigated by the recently proposed structural synthon approach. [2][3][4][5] The structural synthon approach closely parallels the methodology of the supramolecular synthon [6] (Table 1). A single layer, also called "structural synthon" is identified in the place of a molecule, as a repeating unit.…”

“…The structure of bayerite is closest to that of mineral brucite, Mg(OH) 2 . [19] of Al 3+ cation brings about six Al-O distances (coordination symmetry, C 1 ).…”