Cation Substitution in Brucite-like Layers

Possible Identity of Ions and Range of Values of the Ratio 

The basic structure of an LDH may be derived by substitution of a fraction of the divalent cations in a brucite lattice by trivalent cations such that the layers acquire a positive charge, which is balanced by intercalation of anions (and, usually, water) between the layers. It is the possibility of varying the identity and relative proportions of the di- and trivalent cations as well as the identity of the interlayer ions that gives rise to the large variety of materials having the general formula [M S xM x(OH)2] [A ] c/ yH20, which 

A number of empirical studies have attempted to define the boundaries that indicate which metal ions can form LDHs. For example, the roles of the absolute values of ionic radii and the difference in ionic radii of m and [3,17-21] as well as the values of the solubiUty products of m (OH)2 and M COs [22] have been speculated on in the literature, hi earlier publications [1,23], it was often stated that ions such as Cu that are subject to a Jahn-Teller distortion do not form LDHs unless diluted by other cations [24], but more recently examples of Cu -containing LDHs such as [Cuo.69Cro.3i(OH)2]Clo,3i O.6IH2O [25,26] have been well characterized (see Sect. 3.2.2), although the material does show a corrugation of the sheets associated with the Jahn-Teller distortion around individual Cu centers as discussed in Sect. 3.4. 

Theoretical calculations [43] based on first principles molecular dynamics discussed in Sect. 3.2.6 have suggested that Mg Al LDHs are most stable for n = 3 (i.e. x = 0.25) and indeed many minerals, including hydrotalcite itself, have this stoichiometry [4]. It has been reported that the synthesis of LDHs (with benzoate or terephthalate anions in the interlayers) from solutions containing Mg/Al = 2, leads to LDHs having the same composition when the synthesis is carried out at moderate temperatures but LDHs with Mg/Al = 3 (plus AlOOH) when the reaction is carried out under hydrothermal conditions [44]. It was proposed that the latter ratio represents the thermodynamically most favorable product. A similar observation has been reported [45] for solutions with Ni VPe = 2, where hydrothermal preparation led to segregation of an LDH with Ni VPe = 3 and Ni Fe 204. An attempt to synthesize a Co sAl LDH resulted in partial oxidation of the Co and formation of a Co o.yCo o.s LDH with complete migration of Al from the layers to generate interlayer aluminum oxy-species [46]. 

There have been many claims that LDHs can be formed with stoichiometries outside the range 0.20 x 0.33 [17,21], with reported values of x down to as low as 0.07 for Mg/Ga-COs LDHs [47] and as high as 0.41-0.48 for 

---