Thermally Induced Polytype Transformations among the Layered Double Hydroxides (LDHs) of Mg and Zn with Al

The hydrotalcite-like layered double hydroxide (LDH) of Mg with Al shows dramatic changes in the peaks arising from the (h0l)/(0kl) family of reflections in its powder X-ray diffraction pattern during thermal treatment. DIFFaX simulations show that these changes arise due to the transformation of the disordered 3R1 polytype into the 1H polytype on dehydration. The 1H polytype is an essential precursor to the decomposition reaction, which results in the formation of an oxide residue with the rock salt structure. In contrast, the LDH of Zn with Al does not undergo any such transformation, retaining the structure of the 3R1 polytype until decomposition into the oxide residue. Given the poor octahedral site preference of the Zn2+ ion, the 1H polytype is neither structurally stable nor is it topochemically necessary for the thermal decomposition of the Zn−Al LDH, the end product of the decomposition reaction being an oxide with the wurtzite structure

Aging of trivalent metal hydroxide/oxide gels in divalent metal salt solutions: Mechanism of formation of layered double hydroxides (LDHs)

While the aging of freshly precipitated Al(OH)3 gels in solutions of Mg and Ni salts leads to LDH formation at high (> 12) pH, aging of 'Fe(OH)3' leads to LDH formation in Mg salt solutions but not in Ni salt solutions. 'Cr(OH)3' gels do not form LDHs on aging in any of the divalent metal salts. In general, conditions that promote the re-dissolution of the trivalent hydroxide also promote LDH formation showing that oxoanionic species such as AlO2- have a role in LDH formation

Oxidative leaching of chromium from layered double hydroxides: Mechanistic studies

The layered double hydroxide (LDH) of Zn with Cr on treatment with a hypochlorite solution releases chromate ions as a result of oxidative leaching by a dissolution-reprecipitation mechanism. The residue is found to be ε-Zn(OH)2. The LDH of Mg with Cr on the other hand is resistant to oxidative leaching. In contrast, a X-ray amorphous gel of the coprecipitated hydroxides of Mg and Cr yields chromate ions. These results suggest that the oxidation potential of Cr(III) in LDHs is determined by the nature of the divalent ion and the crystallinity of the phase while being unaffected by the nature of the intercalated anions

Conservation of order, disorder, and "crystallinity" during anion-exchange reactions among Layered Double Hydroxides (LDHs) of Zn with Al

Carbonate and chloride ions mediate an ordered stacking of metal hydroxide slabs to yield ordered layered double hydroxides (LDHs) of Zn with Al, by virtue of their ability to occupy crystallographically well-defined interlayer sites. Other anions such as ClO4 - (Td), BrO 3 - (C3v), and NO3 - (coordination symmetry C2v) whose symmetry does not match the symmetry of the interlayer sites (D3h or Oh) introduce a significant number of stacking faults, leading to turbostratic disorder. SO 4 2- ions (coordination symmetry C3v) alter the long-range stacking of the metal hydroxide slabs to nucleate a different polytype. The degree of disorder is also affected by the method of synthesis. Anion-exchange reactions yield a solid with a greater degree of order if the incoming ion is a CO3 2- or Cl-. Incoming NO3 - ions yield an interstratified phase, whereas incoming SO4 2- ions generate turbostratic disorder. Conservation or its converse, elimination, of stacking disorders during anion exchange is the net result of several competing factors such as (i) the orientation of the hydroxyl groups in the interlayer region, (ii) the symmetry of the interlayer sites, (iii) the symmetry of the incoming ion, and (iv) the configuration of the anion. These short-range interactions ultimately affect the long-range stacking order or "crystallinity" of the LDH. © 2007 American Chemical Society

Order and disorder among the layered double hydroxides: combined Rietveld and DIFFaX approach

A combined approach using the Rietveld technique of structure refinement and DIFFaX simulations of the powder patterns enables us to not only arrive at the complete structure of the layered double hydroxides (LDHs), but also classify and quantify the nature of structural disorder. Hydrolysis of urea dissolved in mixed-metal salt solutions containing a divalent metal (Mg 2+, Co2+) with Al3+ results in the homogeneous precipitation of the corresponding LDH. The products obtained are highly crystalline enabling a complete structure determination including subsequent refinement by the Rietveld method. In contrast, the LDH of Ni2+ with Al3+ crystallizes with the incorporation of stacking faults. A combined Rietveld-DIFFaX approach shows that even crystalline samples of this LDH incorporate up to 9 of stacking faults, which are not eliminated even at elevated temperatures (473 K). These studies have implications for the order, disorder and crystallinity of layered phases in general and metal hydroxides in particular. © International Union of Crystallography 2007

DIFFaX simulations of stacking faults in layered double hydroxides (LDHs)

Carbonate-intercalated layered double hydroxides of Co(II) and Ni(II) with Fe(III) and Al(III) were precipitated under different conditions (pH = 8-12; T= 25-80°C). All the samples are replete with stacking faults which are not eliminated by post-precipitation hydrothermal treatment (80-180°C, 18 h). DIFFaX simulations show that the layer stacking sequence of the disordered samples can be generated by a mixture of motifs corresponding to the 3R1, and 2H1, polytypes. These specific sequences are selected in preference to others because of the need for hydrogen bonding between the intercalated carbonates and hydroxide sheets. Thermodynamic considerations show that faulted crystals have greater stability than ordered crystals. Stacking faults arising from a mixture of 3R1, and 2H1 motifs, while having the same enthalpy as that of the ordered crystal, nevertheless contribute to thermodynamic stability by enhancing disorder. Copyright © 2005, The Clay Minerals Society

Mechanism of the anion exchange reactions of the layered double hydroxides (LDHs) of Ca and Mg with Al

The nitrate containing layered double hydroxide (LDH) of Ca with Al on reaction with aqueous solutions of Na2CO3 and Na 3PO4 yields CaCO3 and Ca5(PO 4)3OH respectively rather than the carbonate/phosphate containing LDHs. The LDH of Mg with Al on reacting with dissolved phosphate ions also leads to the formation of unitary phosphates. This shows that the anion exchange reactions of layered double hydroxides take place by the dissolution-reprecipitation mechanism rather than by the topotactic mechanism. © 2005 Elsevier SAS. All rights reserved

Disorder in layered hydroxides: Synthesis and DIFFaX simulation studies of Mg(OH)2

The characteristics of Mg(OH)2 precipitated under different conditions were studied by using powder X-ray diffraction (PXRD) technique. The compound was found to exibit characteristic non-uniform broadening of Bragg peaks. The study showed that the broadening arised due to interstratification and turbostratic disorder

Solution decomposition of the layered double hydroxide (LDH) of Zn with Al

The layered double hydroxides (LDH) of Zn with Al containing intercalated CO32− and NO3− ions undergo solution decomposition to yield a highly crystalline oxide mixture comprising ZnO and ZnAl2O4 at temperatures as low as 150–180 °C under hydrothermal conditions. In contrast solid-state decomposition takes place at a much higher temperature (240–315 °C) in air. Solution decomposition is not only guided by the low octahedral crystal field stabilization energy of Zn2+ ions, a factor that also affects solid-state decomposition, but also by solubility considerations. The LDHs of Mg and Ni with Al do not undergo solution decomposition

Suppression of spinel formation to induce reversible thermal behavior in the layered double hydroxides (LDHs) of Co with Al, Fe, Ga, and In

The layered double hydroxides (LDHs) of Co with trivalent cations decompose irreversibly to yield oxides with the spinel structure. Spinel formation is aided by the oxidation of Co(II) to Co(III) in the ambient atmosphere. When the decomposition is carried out under N2, the oxidation of Co(II) is suppressed, and the resulting oxide has the rock salt structure. Thus, the Co-Al-CO3 2-/Cl- LDHs yield oxides of the type Co1-xAl2x/3x/3O, which are highly metastable, given the large defect concentration. This defect oxide rapidly reverts back to the original hydroxide on soaking in a Na2CO3 solution. Interlayer NO3 - anions, on the other hand, decompose generating a highly oxidizing atmosphere, whereby the Co-Al-NO3 - LDH decomposes to form the spinel phase even in a N2 atmosphere. The oxide with the defect rock salt structure formed by the thermal decomposition of the Co-Fe-CO3 2- LDH under N2, on soaking in a Na2CO3 solution, follows a different kinetic pathway and undergoes a solution transformation into the inverse spinel Co(Co,Fe)2O4. Fe3+ has a low octahedral crystal field stabilization energy and therefore prefers the tetrahedral coordination offered by the structure of the inverse spinel rather than the octahedral coordination of the parent LDH. Similar considerations do not hold in the case of Ga- and In-containing LDHs, given the considerable barriers to the diffusion of M3+ (M=Ga, In) from octahedral to tetrahedral sites owing to their large size. Consequently, the In-containing oxide residue reverts back to the parent hydroxide, whereas this reconstruction is partial in the case of the Ga-containing oxide. These studies show that the reversible thermal behavior offers a competing kinetic pathway to spinel formation. Suppression of the latter induces the reversible behavior in an LDH that otherwise decomposes irreversibly to the spinel. © 2007 American Chemical Society