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Suppression of the Reversible Thermal Behavior of the Layered Double Hydroxide (LDH) of Mg with Al: Stabilization of Nanoparticulate Oxides

Radha, AV and Kamath, Vishnu P and Ravishankar, N and Shivakumara, C (2007) Suppression of the Reversible Thermal Behavior of the Layered Double Hydroxide (LDH) of Mg with Al: Stabilization of Nanoparticulate Oxides. In: Langmuir, 23 (14). pp. 7700-7706.

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Abstract

The layered double hydroxide of Mg with Al decomposes below 600°C with the loss of nearly 48% mass, resulting in the formation of an oxide residue having the rock salt structure and nanoparticulate morphology. However, this product reconstructs back into the parent LDH, owing to its compositional and morphological metastability. The oxide can be kinetically stabilized within an amorphous phosphate network built up through an ex situ reaction with a suitable phosphate source such as $(NH_4)H_2PO_4$. This oxide transforms into a thermodynamically more stable phase with a spinel structure on soaking in an aqueous medium. The oxide residue has a nanoparticulate morphology as revealed by the Scherrer broadening of the Bragg reflections as well as by electron microscopy. This work shows that the hydroxide reconstruction reaction and spinel formation are competing reactions. Suppression of the former catalyzes spinel formation as the excess free energy of the metastable oxide residue is unlocked to promote the diffusion of $Mg^{2+}$ ions from octahedral to tetrahedral sites, which is the essential precondition to the formation of a normal spinel. This reaction taking place as it does at ambient temperature and in solution helps in the retention of a nanostructured morphology for the spinel. Another way of stabilizing the oxide is by incorporating the thermally stable borate anion into the LDH. This paves the way for an in situ reaction between the cations of the host LDH and the borate guest. The in situ reaction directly leads to the formation of an oxide with a spinel structure.

Item Type: Journal Article
Additional Information: Copyright of this article belongs to American Chemical Society(ACS).
Department/Centre: Division of Chemical Sciences > Materials Research Centre
Division of Chemical Sciences > Solid State & Structural Chemistry Unit
Date Deposited: 21 Sep 2007
Last Modified: 19 Sep 2010 04:39
URI: http://eprints.iisc.ernet.in/id/eprint/11860

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