Cationic Order and Structure of [Zn−Cr−Cl] and [Cu−Cr−Cl] Layered Double Hydroxides: A XRD and EXAFS 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 -3 m 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 Cu 2+ octahedra, which is opposite to the [Zn−Cr−Cl] sheets presenting a relative flatness.