Layered double hydroxides

Layered double hydroxides (LDH) are a class of ionic solids characterized by a layered structure with the generic layer sequence [AcB Z AcB]n, where c represents layers of metal cations, A and B are layers of hydroxide (OH-) anions, and Z are layers of other anions and neutral molecules (such as water). Lateral offsets between the layers may result in longer repeating periods. The intercalated anions (Z) are weakly bound, often exchangeable; their intercalation properties have scientific interest and industrial applications. LDHs occur in nature as minerals, as byproducts of metabolism of certain bacteria, and also unintentionally in man-made contexts, such as the products of corrosion of metal objects. LDHs can be seen as derived from hydroxides of divalent cations (d) with the brucite (Mg(OH)2) layer structure [AdB AdB]n, by cation (c) replacement (Mg2+ → Al3+), or by cation oxidation (Fe2+ → Fe3+ in the case of green rust, Fe(OH)2), in the metallic divalent (d) cation layers, so as to give them an excess positive electric charge; and intercalation of extra anion layers (Z) between the hydroxide layers (A,B) to neutralize that charge, resulting in the structure [AcB Z AcB]n. LDHs can be formed with a wide variety of anions in the intercalated layers (Z), such as Cl−, Br−, NO, CO, SO and SeO. This structure is unusual in solid state chemistry, since many materials with similar structure (such as montmorillonite and other clay minerals) have negatively charged main metal layers (c) and positive ions in the intercalated layers (Z). In the most studied class of LDHs, the positive layer (c) consists of divalent and trivalent cations, and can be represented by the formula [M(II)1-xM(III)x(OH-)2]x+ [(Xn−)x/n · yH2O]x-, where Xn− is the intercalating anion (or anions). Most commonly, M(II) = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+, and M(III) is another trivalent cation, possibly of the same element. Fixed-composition phases have been shown to exist over the range 0.2 ≤ x ≤ 0.33.