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 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 R3[combining macron]m, 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, D3h) select for the 3R1 polytype, the XO3(-) (X = Br, I) ions (molecular symmetry, C3v) select for the rare 3R2 polytype. This work demonstrates the role of the intercalated anion in structure selection of the LDH.

Electronic‐Structure Calculations of Cation‐Ordered II‐III Layered Double Hydroxides: Origin of the Distortion of the Metal‐Coordination Symmetry

“…All of the three LDHs crystallize in hexagonal symmetry (space group P 3; a obs = 5.35–5.41 Å, c obs = 11.05–11.15 Å; obs: observed) . These structures were used as input for the SIESTA program.…”

“…One of the significant structural features that distinguishes a cation‐ordered structure is the reduction in the symmetry of the [M(OH) 6 ] polyhedron. In a (pseudo) single‐cation structure model, which describes a cation‐disordered structure, the idealized coordination symmetry around the metal ions is D 3 d . We ask the question: What are the possible factors underlying the observed distortion?…”

“…(iii) The [M(OH) 6 ] (M = Cu 2+ , Zn 2+ ) coordination symmetry is lowered from the ideal D 3 d to C 3 (or ca. C 3 v ), whereas the coordination symmetry of [M′(OH) 6 ] (M′ = Al 3+ , Cr 3+ ) is preserved as D 3 d , which is expected for the single‐cation structure …”

“…The models for the calculations were generated on the basis of the reported crystal structures for [Zn–Al], [Cu–Cr], and [Zn–Cr] LDHs , . The cell size (2√3 × 2√3) R30° was chosen with proper three‐dimensional periodic boundary conditions for the calculation.…”

“…Experimental work aimed at the precipitation of cationordered structures has primarily focused on LDHs with the composition of x = 0.33. [17] Evidence for cation ordering manifests itself in the form of weak supercell reflections, corresponding to the 100 and 101 planes of the enlarged unit cell in powder X-ray diffraction patterns. These structures have been refined within the space group P3 (a ≈ 5.35 Å, c ≈ 11.5 Å).…”