Cationic order and structure of [Zn-Cr-Cl] and [Cu-Cr-Cl] layered double hydroxides: A XRD and EXAPS study

X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) were used to evidence a cationic order within sheets of the layered double hydroxides (LDH) [Zn-Cr-Cl] and [Cu-Cr-Cl]. The [Zn-Cr-Cl] LDH is well kown for its ability to be synthesized only with the ratio Zn/Cr = 2, whatever the proportion of di- and trivalent cations in the starting solutions; it would thus be a good candidate for showing cationic order. The XRD diagrams were identical and were indexed in the R-3m space group, no superstructure being observed. So, the cations, (Zn, Cr) and (Cu,Cr) for [Zn-Cr-Cl] and [Cu-Cr-Cl], respectively, are distributed statistically on the same site in the structural model obtained by Rietveld refinement from the XRD diagrams. The interlamellar species (Cl and O of water) show a relative disordered state. Local description of the arrangement around cations by EXAFS at cations K edge concludes unambiguously for [Cu-Cr-Cl] to a cation ordering; strong evidence of similar ordering has been obtained for [Zn-Cr-Cl]. We have also shown that this technique is only sensible to the species of a same sheet. EXAFS simulations performed at Cr and Cu K edge on the [Cu-Cr-Cl] LDH also show a corrugation in its sheets because of the Jahn-Teller distortion in Cu2+ octahedra, which is opposite to the [Zn-Cr-Cl] sheets presenting a relative flatness.