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L. Storaro Istituto di Chimica, via del Cotonificio 108, 33100 Udine, Italy

R. Frattini and S. Enzo Dipartimento di Chimica Fisica, DD 2137, 30123 Venezia, Italy (Received 5 March 1992; accepted 13 May 1992)

Finely dispersed metal powders have been obtained after chemical reduction of Ni and Co acetylacetonate by lithium aluminum hydride in tetrahydrofuran at low temperature. The Al/Ni and Al/Co stoichiometry of the as-reduced powders was 1.1 and 1.2, respectively. The structure and thermal stability of the as-reduced powders were affected by the temperature of reduction. For the NiAl powders it was found that the thermal treatment initially induces a separation of highly unstable Ni(Al) and Al(Ni) solid solutions, which subsequently react to give a single NiAl phase of cubic structure not reported in the equilibrium phase diagram. Conversely, the reduction of cobalt acetylacetonate directly gives a cubic metastable phase, from which precipitates some hexagonal form of Co after treatment at 450 °C.

I. INTRODUCTION There is growing interest in the potential utility of chemical synthesis as a means of preparing materials for high technology applications.1 Highly dispersed metal powders have aroused great interest for their application in chemical synthesis as well as their use in materials science.2 Numerous methods exist for the preparation of metal powders.3 In a series of papers beginning in 1972 the Rieke group described the preparation of highly reactive metal powders in organic solvents.4 Transition metal powders prepared by these methods often showed unusual chemical properties, but few data are available on their structural properties. Since their discovery many years ago both sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4) have become the pre-eminent reducing agents of modern organic chemistry.5 More recently the use of transition metal catalysts as co-reagents has extended the use of these compounds as reducing agents. The most widely used catalytic systems are those containing nickel and cobalt salts.6 Little is known about the actual mechanism of these transformations grouped together as "transition metal assisted" hydride reductions. The reaction between nickel and cobalt salts and alkali metal-boron hydrides invariably produces black precipitates that were structurally characterized only recently as an amorphous phase of nickel and cobalt borides.8 The nickel or cobalt aluminum system obtained by reduction of metallic salts with LiAlH4 is the less known. A recent study7 on the mechanism of these transition metal catalyzed reductions showed that the 2418

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J. Mater. Res., Vol. 7, No. 9, Sep 1992

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finely divided black powder obtained from the reaction is an intermetallic compound, whose composition depends slightly on the nature of the solvent, on the metallic salts, and on the reaction conditions employed. It was found that the reaction between homogeneous solutions of metallic salts and lithium aluminum hydride gives the most rep