Some layered hydroxides can also undergo isomorphic substitution of hydroxyl groups by other oxo-ions or by water molecules. In the last case, additional anions will be required to neutralize the excess of positive charge in the layers, keeping the cations unaltered, i. e. only divalent cations are present in the layers. The resulting compounds are called layered hydroxide salts. According to this description, LHSs can be classified based on the structure of copper hydroxide nitrate—Cu2(0HLN03—and zinc hydroxide nitrate—Zn5(0H)g(N03L • 2H2O. The general formula for an LHS is M2+(0H)2_x(A”~)x=n$mH20,

where M = Mg, Ni, Zn, Cu, Co and A = N03~, S0|~ e Cl~ (Arizaga et al., 2007).

The layers in the copper hydroxide nitrate structure are formed by octahedrons whose center is occupied by Cu2+ cations and these are coordinated to hydroxyl groups and nitrate ions that have substituted M of the hydroxyl sites. This example is the easiest description of an LHS.

The structure of zinc hydroxide nitrate has two main characteristics. The first is that M of the Zn2+ cations in octahedral coordination with hydroxyl groups migrate out of the layers, leaving empty octahedrons and form­ing tetrahedrons up and down the empty octahedral sites. Then, each layer is formed by zinc cations in octa­hedral coordination with hydroxyl groups and in tetra­hedral coordination, whose base is formed by three hydroxyl groups shared with the main octahedral layer and its apex is occupied by water molecules. The result­ing layers have residual positive charge with com­position [ZnoctZn2etr(OH)g(H2O)2]2+, where oct and tetr indicate octahedral and tetrahedral sites (Stahlin and Oswald, 1971).

The residual positive charge in the layer is neutral­ized with nitrate ions in the interlayer space in a perpen­dicular position to the layers plane. Normally, nitrate ions do not coordinate directly to the cations in the layers; however, part of the nitrate ions can be grafted to the layers by controlling the pH of the synthesis (Arizaga et al., 2008). The stacking of layers is stable because of the numerous hydrogen bonds that are formed between OH groups in the layer, nitrate ions and the interlayer water molecules. Two adjacent layers are shifted by a factor of b/2 along the (001) plane and are stacked along the basal axis.