Observation of cation ordering and anion-mediated structure selection among the layered double hydroxides of Cu(II) and Cr(III)

Highly ordered Cl- and SO42--intercalated layered double hydroxides (LDHs) of Cu(II) and Cr(III) are obtained when coprecipitation is carried out at low pH similar to 5 and elevated temperature (60-80 degrees C). Precipitation under other conditions results in the formation of a gel. The SO42--LDH exhibits weak reflections which could be indexed to the 100 and 101 planes of a supercell corresponding to a = root 3 x a(o), providing direct evidence for cation ordering among LDHs by X-ray diffraction. The ordering of the M(II) and M'(III) in the metal hydroxide layer has been a subject of considerable debate in the LDH literature for the past several years and was earlier probed using short-range techniques such as NMR and EXAFS. Rietveld refinement indicates that the cation-ordered LDH adopts the structure of the 1H polytype (space group P (3) over bar, a = 5.41 angstrom, c = 11.06 angstrom). In contrast the Cl--intercalated LDH adopts the cation-disordered structure of the 3R(1) polytype (space group R (3) over barm, a = 3.11 angstrom, c = 23.06 angstrom). The Cl--LDH was used as a precursor to synthesize LDHs with other anions. While Br- and CO32- (molecular symmetry, D-3h) select for the 3R(1) polytype, the XO3- (X = Br, I) ions (molecular symmetry, C-3v) select for the rare 3R(2) polytype. This work demonstrates the role of the intercalated anion in structure selection of the LDH.