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

摘要

Highly ordered Cl- and SO_4 ~(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 SO_4 ~(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 ?, c = 11.06 ?). In contrast the Cl--intercalated LDH adopts the cation-disordered structure of the 3R_1 polytype (space group R3m, a = 3.11 ?, c = 23.06 ?). The Cl~--LDH was used as a precursor to synthesize LDHs with other anions. While Br- and CO_3 ~(2-) (molecular symmetry, D_(3h)) select for the 3R1 polytype, the XO_3 ~- (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.