Thermal Properties of Terephthalate- and Benzoate-Intercalated LDH

Cation Ordering in Synthetic Layered Double Hydroxides

“…It is interesting to note that although close to the nominal values of 1:2, 1:3 and 1:4, respectively, the cation ratios always tend towards 1:3. This is consistent with the observation that natural LDH minerals predominantly have a 1:3 III:II cation ratio and the authors' hypothesis that longer and fiercer synthetic conditions drive hydrotalcites towards a 1:3 AI:Mg ratio (Vucelic et al 1995).…”

“…This very great similarity in all the patterns immediately suggests that there is no long-range Fe(III) cation ordering (that is, ordering on a length scale beyond a few tens of angstroms), since if there were any the unit cell size and shape would have to change as the Fe loading varied. This is confirmed by the fact that all peaks can be assigned using a unit cell based on the shortest cation--cation distance; that is, (Bookin et al 1993;Vucelic et al 1995). In our case, for the 1:2 Fe:Mg pyroaurite, one expects a superlattice with unit cell of ~"3a, corresponding to a reflection at about 19.0 ~ (d = 4.67 ,~), if Fe cations never neighbor each other.…”

“…This is an interesting result in view of the fact that in nature pyroaurites usually seems to occur in the 1:3 Fe:Mg form (Allmann 1968;Drits et al 1987) and that there is some evidence that hydrothermal synthesis at elevated temperature and pressure results in the formation of 1:3 Fe:Mg pyroaurite from 1:2 mother liquor (Vucelic et al 1995). The Debye-Waller factor described above gives some support to the idea that, at high concentrations, the trivalent cations result in greater distortion of the lattice, resulting in more static disorder, and this would fit well with the 1:2 Fe:Mg form being destabilized.…”

“…Diffraction probes the long-range order (tens of angstroms and more) in crystalline materials and thus allows one to establish whether there is a highly ordered 2-dimensional superlattice present. Such superlattices have been observed by powder XRD previously in the Li/A1 LDHs, in which the Li ions fill vacancies in a gibbsitelike structure and so naturally form a long-range superlattice (Serna et al 1982;Sissoko et al 1985;Dutta and Puri 1988), and as a result of anion ordering of sulphate (Bookin et al 1993) or benzoate (Vucelic et al 1995) in the interlayer of hydrotalcites, where geometric factors force a high degree of ordering into the interlayer region. By contrast, XAS probes the local structure around an absorbing atom and so allows observation of short-range ordering (up to about 6-8 ,~) in crystalline or amorphous materials; thus it is ideal when looking for effects such as triply charged cations not sitting in neighboring sites within brucite-like layers.…”

“…It is interesting that although diffraction features from cation ordering in M(II)/M(III) LDHs have never been observed, reflections resulting from the ordering of anions within the interlayer region have been reported, both in the case of LDH benzoate and sulphate (Bookin et al 1993;Vucelic et al 1995). Thus there must be a mismatch between the positions of positive charges in the brucite-like layers and the anions balancing those charges.…”