A Crystal Chemical Approach to a Cation-​Ordered Structure Model for Carbonate-​Intercalated Layered Double Hydroxides

Layered double hydroxides comprise a stacking of pos. charged metal hydroxide layers with anions and water mols. included in the interlayer galleries. Among the anions, the carbonate ion is the most ubiquitous in both mineral and lab. synthesized phases. Taylor (1973) suggested that the carbonate ion (mol. symmetry D3h) prefers a trigonal prismatic interlayer site (local symmetry D3h)​, whereby the hydrogen bonding with the metal hydroxide layer is maximized. However, the cation ordered structure models of hexagonal symmetry include interlayer sites which are exclusively trigonal antiprisms (local symmetry D3d)​. In keeping with Taylor's criterion, a hexagonal stacking of metal hydroxide layers does not permit the inclusion of carbonate ions in the interlayer. In this work, a crystal chem. approach is adopted based on the translationgleiche subgroups of hexagonal and cubic summits to arrive at a structure model based on the space group C2​/m. In this structure, not only is the 3-​fold symmetry of metal coordination retained, but also interlayer sites of ∼D3h symmetry are generated to host the intercalated carbonate ions. Using this model, the structures of a cohort of carbonate-​intercalated layered double hydroxides are refined

A Crystal Chemical Approach to a Cation-Ordered Structure Model for Carbonate-Intercalated Layered Double Hydroxides

Layered double hydroxides comprise a stacking of positively charged metal hydroxide layers with anions and water molecules included in the interlayer galleries. Among the anions, the carbonate ion is the most ubiquitous in both mineral and laboratory synthesized phases. Taylor (1973) suggested that the carbonate ion (molecular symmetry D3h) prefers a trigonal prismatic interlayer site (local symmetry D3h), whereby the hydrogen bonding with the metal hydroxide layer is maximized. However, the cation ordered structure models of hexagonal symmetry include interlayer sites which are exclusively trigonal antiprisms (local symmetry D3d). In keeping with Taylor’s criterion, a hexagonal stacking of metal hydroxide layers does not permit the inclusion of carbonate ions in the interlayer. In this work, a crystal chemical approach is adopted based on the translationgleiche subgroups of hexagonal and cubic summits to arrive at a structure model based on the space group C2/m. In this structure, not only is the 3-fold symmetry of metal coordination retained, but also interlayer sites of ∼D3h symmetry are generated to host the intercalated carbonate ions. Using this model, the structures of a cohort of carbonate-intercalated layered double hydroxides are refined

Electrodeposition of Dicalcium Phosphate Dehydrate Coatings on Stainless Steel Substrates

Cathodic reduction of an aqueous solution containing dissolved calcium and phosphate ions results in the deposition of micrometer thick CaHPO4·2H2O (dicalcium phosphate dihydrate) coatings on stainless steel substrates. The coating obtained at a low deposition current (8 mA cm-2) comprises lath-like crystallites oriented along 020. The 020 crystal planes are non-polar and have a low surface energy. At a high deposition current (12 mA cm-2), platelets oriented along 12¯1 are deposited. CaHPO4·2H2O is an important precursor to the nucleation of hydroxyapatite, the inorganic component of bones. Differently oriented CaHPO4·2H2Ocoatings transform to hydroxyapatite with different kinetics, the transformation being more facile when the coating is oriented along 12¯1. These observations have implications for the development of electrodeposited biocompatible coatings for metal endoprostheses for medical applications

Thermal, solution and reductive decomposition of Cu-Al layered double hydroxides into oxide products

Cu-Al layered double hydroxides (LDHs) with Cu/Al ratio 2 adopt a structure with monoclinic symmetry while that with the ratio 0.25 adopt a structure with orthorhombic symmetry. The poor thermodynamic stability of the Cu-Al LDHs is due in part to the low enthalpies of formation of Cu(OH)2 and CuCO3 and in part to the higher solubility of the LDH. Consequently, the Cu-Al LDH can be decomposed thermally (150 °C), hydrothermally (150 °C) and reductively (ascorbic acid, ambient temperature) to yield a variety of oxide products. Thermal decomposition at low (400 °C) temperature yields an X-ray amorphous residue, which reconstructs back to the LDH on soaking in water or standing in the ambient. Solution decomposition under hydrothermal conditions yields tenorite at 150 °C itself. Reductive decomposition yields a composite of Cu2O and Al(OH)3, which on alkali-leaching of the latter, leads to the formation of fine particles of Cu2O (<1 μm). © 2009 Elsevier Inc. All rights reserved

Synthesis, Structure Refinement and Chromate Sorption Characteristics of an Al-Rich Bayerite-Based Layered Double Hydroxide

“Imbibition” of Zn2+ ions into the cation vacancies of bayerite–Al(OH)3 and NO3- ions into the interlayer gallery yields an Al-rich layered double hydroxide with Al/Zn ratio ~3. NO3- ions are intercalated with their molecular planes inclined at an angle to the plane of the metal hydroxide slab and bonded to it by hydrogen bonds. Rietveld refinement of the structure shows that the monoclinic symmetry of the precursor bayerite is preserved in the product, showing that the imbibition is topochemical in nature. The nitrate ion is labile and is quantitatively replaced by CrO42- ions from solution. The uptake of CrO42- ions follows a Langmuir adsorption isotherm, thus showing that the hydroxide is a candidate material for green chemistry applications for the removal of CrO42- ions from waste water. Rietveld refinement of the structure of the hydroxide after CrO42- inclusion reveals that the CrO42- ion is intercalated with one of its 2-fold axes parallel to the b-crystallographic axis of the crystal, also the principal 2 axis of the monoclinic cell

Layered double hydroxides of Ni with Cr and Mn as candidate electrode materials for alkaline secondary cells

The use of layered double hydroxides (LDH) of Ni with Cr and Mn as electrode materials for alkaline secondary cells was studied. The presence of Cr and Mn suppresses β-nickel hydroxide formation which indicates their strong influence on the precipitation behavior of Ni2+. It is found that the LDH of Ni with Cr and Mn are electrochemically active and deliver capacities of 500 (Ni-Cr), 400 (Ni-Mn, x = 0.2) and 430 (Ni-Mn, x = 0.1) mAhg-1 of Ni respectively

Effect of Hydration on Polytypism and Disorder in the Sulfate-Intercalated Layered Double Hydroxides of Li and Al

The double hydroxide of Li+ and Al3+ is an anionic clay comprising positively charged metal hydroxide layers and intercalated anions. While the structure of the iono-covalently bonded metal hydroxide layer is well known, relatively less knowledge is available regarding the manner in which the anions and water molecules are packed in the interlayer region. The sulfate ion is of special interest as it can potentially intercalate in a multiplicity of orientations and grow an extended hydration sphere. The sulfate-intercalated double hydroxide was synthesized by the imbibition of Li2SO4 into both the gibbsite and bayerite forms of Al(OH)3 to obtain layered double hydroxides with the nominal formula Li2Al4(OH)12SO4·nH2O (n = 4–8). The as-prepared compounds were poorly ordered and did not yield any structural information. Temperature-induced partial dehydration yielded ordered phases of different structures in the two systems. Simulation of the powder patterns of different model structures, followed by structure refinement in both direct and reciprocal spaces, showed that the gibbsite-derived phase yielded a two-layer polytype of hexagonal symmetry (space group P63/m). The local symmetry of the sulfate ion was close to D 2d with one of the C 2 axes of the SO4 2- being nearly parallel to the c axis of the crystal. The bayerite-derived phase yielded a one-layer polytype of monoclinic symmetry (space group C2/m). The sulfate ion was oriented with its C 3 axes tilted away from the stacking direction. Cooling and rehydration (relative humidity ∼70%) resulted in a reversible expansion of the basal spacing due to the ingress of water molecules from the ambient humidity into the interlayer region. Hydration in both cases resulted in turbostratic disorder. The disorder in the bayerite-derived phase was a result of random intergrowth of motifs with rhombohedral and monoclinic symmetries

Energy storage and retrieval: the secondary battery route

Harnessing sunlight for the production of electrical energy is an engrossing prospect. The crucial concept underlying the success of solar power stations is energy storage and its retrieval on demand which can be most effectively achieved with storage batteries. This article highlights the chemistry of existing and emerging battery technologies

Cyclic voltammetric studies of pure and doped films of cobalt hydroxide in 1M potassium hydroxide

Cyclic voltammetric studies of electrosynthetically prepd. pure and doped thin films of cobalt hydroxide (on a platinum foil substrate) in 1M KOH show that Co(OH)​2 undergoes a quasi-​reversible redox reaction between a nearly quadrivalent black oxidic phase and a trivalent brown bronze-​like phase. The coulombic efficiency is 60​%. A common scheme is proposed for the electrode reactions of oxide electrodes in alk. media, namely: MII(OH)​2 .dblharw. HMIIIO2 .dblharw. MIVO2, where M = Mn, Co, Ni. The three metals execute different steps in this two-​electron redox reaction, either reversibly or qu..

Reversible hydration and aqueous exfoliation of the acetate-intercalated layered double hydroxide of Ni and Al: Observation of an ordered interstratified phase

Acetate-intercalated layered double hydroxides (LDHs) of Ni and Al undergo reversible hydration in the solid state in response to the ambient humidity. The LDH with a high layer charge (0.33/formula unit) undergoes facile hydration in a single step, whereas the LDH with a lower layer charge (0.24/formula unit) exhibits an ordered interstratified intermediate, comprising the hydrated and dehydrated layers stacked alternatively. This phase, also known as the staged S-2 phase, coexists with the end members suggesting the existence of a solution-type equilibrium between the S-2 phase and the end members of the hydration cycle. These LDHs also undergo facile aqueous exfoliation into 2-5 nm-thick tactoids with a radial dimension of 0.2-0.5 μm. © 2012 Elsevier Inc. All rights reserved