Handbook of Clay Science Edited by F. Bergaya, B.K.G. Theng and G. Lagaly Developments in Clay Science, Vol. 1 r 2006 Published by Elsevier Ltd.
LAYERED DOUBLE HYDROXIDES C. FORANOa, T. HIBINOb, F. LEROUXa AND C. TAVIOT-GUE´HOa a
Laboratoire de Mate´riaux Inorganiques, CNRS UMR 6002, Universite´ Blaise Pascal, F-63177 Aubie`re Cedex, France b Ecological Materials Group, AIST, 16-1 Onogawa, Tsukuba 305-8569, Japan
13.1.1. DEFINITIONS Among the group of minerals referred to as ‘Non-Silicate Oxides and Hydroxides’ (Newman, 1987), the ‘layered double hydroxides’ (LDH) have many physical and chemical properties that are surprisingly similar to those of clay minerals. Their layered structure, wide chemical compositions (due to variable isomorphous substitution of metallic cations), variable layer charge density, ion-exchange properties, reactive interlayer space, swelling in water, and rheological and colloidal properties make LDH clay-like. But because of their anion-exchange properties, LDH were referred to as ‘anionic clays’. As hydrotalcite is one of the most representative mineral of the group, LDH were also called ‘hydrotalcite-like compounds’ (HTlc). The structure of hydrotalcite is related to that of brucite, Mg(OH)2 in which some of the Mg2+ cations in the layer structure were replaced by Al3+. Carbonate anions are intercalated between the layers to maintain electroneutrality. The chemical formula of hydrotalcite may therefore be given as Mg0.75Al0.25(OH)2(CO3)0.5 0.5H2O, and abbreviated to [Mg–Al–CO3] or [Mg–Al]. The general formulae for other members of the family, based on a combination of III q divalent and trivalent metal cations, can be written as [MII 1xM x(OH)2][X x/q II III II III II III II III nH2O] or [M –M –X] or [M –M ], where [M1xM x(OH)2] ([M –M ]) represents the layer, and [Xqx/q nH2O] the interlayer composition (Figs. 13.1.1 and 13.1.2). Extension to multicomponent systems may be expressed as [MII–M0 II–MIII–M00 III–X–Y]. Tetravalent cations such as Zr4+ and Sn4+ can also be incorporated (Velu et al., 1999a). Cation radius (size) is an important parameter in LDH formation. The LDH structure is not stable when the ionic radius of M(II) is o0.06 nm. With large cations such as Ca2+, the hydrotalcite-type structure transforms into that of hydrocalumite. DOI: 10.1016/S1572-4352(05)01039-1