Trivalent cation substitution effect into layered double hydroxides Co2FeyAl1-y(OH)6Cl •nH2O:: Study of the local order -: Ionic conductivity and magnetic properties

A series of layered double hydroxide materials of composition Co2FeyAl1-y (OH)(6)Cl . nH(2)O (0 less than or equal to y less than or equal to 1) was prepared via chimie douce. The crystalline parameter related to the cation to cation distance obeys the expected variation, showing that the substitution is effective over the entire range. Local order around Co and Fe cations is studied by X-ray absorption spectroscopy. Moduli of the Fourier transform at the Fe K-edge are superimposable, in agreement with an ordered model, although present in small domains since no superlattice is depicted. The ionic resistivity of the samples is highly dependent on the water molecule content. The conductivity is found to be thermally assisted, and the variation of the slope in the Arrhenius diagram is explained by a Vogel-Tamman-Fulcher-type behavior. Magnetic susceptibility measurements support the proposed cation composition and indicate the onset of local magnetic order at low temperature (below 10 K). The inter-sheet distance influences the magnetic response at low temperature, showing the presence of weak interactions between lamellae. (C) 2002 Elsevier Science (USA).