A. V. Radha

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.

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 salt3’ gels do not form LDHs on aging in any of the divalent metal salts. In general, conditions that promote the redissolution of the trivalent hydroxide also promote LDH formation showing that oxoanionic species such as AlO-2 have a role in LDH formation.

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

Powder X-ray diffraction (PXRD) patterns of magnesium hydroxides precipitated under different conditions exhibit characteristic non-uniform broadening of Bragg peaks which cannot be explained merely on the basis of Scherrer broadening. The broadening is shown to arise due to interstratification and turbostratic disorder.

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.

BROMIDE-ION DISTRIBUTION IN THE INTERLAYER OF THE LAYERED DOUBLE HYDROXIDES OF Zn AND Al: OBSERVATION OF POSITIONAL DISORDER

Because of the anisotropy in bonding, layered hydroxides crystallize with extensive structural disorder due to the incorporation of stacking faults. In contrast, the loss of crystallinity in Br−-ion intercalated layered double hydroxides (LDHs) arises due to the positional disorder of Br− in the interlayer. The structure of the interlayer in other LDHs is poorly understood due to the low X-ray scattering power of the commonly found anions such as Cl− and

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

{\rm{NO}}_3^ -

Conservation of order, disorder, and crystallinity during anion-exchange reactions among layered double hydroxides (LDHs) of Zn with Al

NO3− relative to that of the metal hydroxide layers. On heating to 175°C, the Br− ion migrates from positions of lesser site degeneracy to those of greater site degeneracy as dehydration of the interlayer opens up access to positions hitherto occupied by intercalated water molecules. The new (18h) site is situated closer to the proton of the metal hydroxide layer (1.809 Å) compared to the 6c site (2.402 Å). This shows a pre-association of the bromide ion with the proton of the hydroxide layer leading to the release of HBr upon decomposition of the bromide-containing LDHs. The fact that Cl−-containing LDHs also decompose with the evolution of HCl shows that such a redistribution of the atoms in the interlayer is more common than is generally recognized.