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

Highly ordered Cl^- and SO₄^2--intercalated layered double hydroxides (LDHs) of Cu(ii) and Cr(iii) are obtained when coprecipitation is carried out at low pH ∼ 5 and elevated temperature (60-80 °C). Precipitation under other conditions results in the formation of a gel. The SO₄^2--LDH exhibits weak reflections which could be indexed to the 100 and 101 planes of a supercell corresponding to a = √3 × 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 P3, a = 5.41 Å, c = 11.06 Å). In contrast the Cl^--intercalated LDH adopts the cation-disordered structure of the 3R₁ polytype (space group R3m, a = 3.11 Å, c = 23.06 Å). The Cl^--LDH was used as a precursor to synthesize LDHs with other anions. While Br^- and CO₃^2- (molecular symmetry, D_3h) select for the 3R₁ polytype, the XO₃^- (X = Br, I) ions (molecular symmetry, C_3v) select for the rare 3R₂ polytype. This work demonstrates the role of the intercalated anion in structure selection of the LDH.