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

Abstract

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 ∼ 5 and elevated temperature (60–80 °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 = √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 P, 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 Rm, 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 CO₃²⁻ (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.