The Synthesis of New Compounds Containing Metal Clusters Using Designed Superlattice Reactants to Control the Kinetics and Reaction Mechanism

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9510562 Johnson This proposal focuses upon the synthesis of new ternary transition metal chalcogenides which contain discrete clusters of transition metal atoms. The ternary molybdenum chalcogenides and the alkali metal intercalates of the fullerines are both examples of this class of compounds, containing discrete but interacting cluster entities. Both of these classes of materials have been found to have a host of unusual properties ranging from catalytic activity to superconductivity which have been rationalized as resulting from the unique cluster structure found in these materials. The goal is to prepare tungsten, niobium and tantalum compounds which are isostructural analogs of the ternary molybdenum chalcogenides. The preparation of isostructural and isoelectronic compounds to the ternary molybdenum compounds will permit the testing of these structure based arguments and will hopefully extend the range of properties found in this unique class of materials which is some sense bridges the gap between discrete molecular compounds and extended structures . %%% Chemical reactions involving solids at low temperatures are very sluggish even though there is often a large driving force for the reactions. Previous attempts to prepare new ternary transition metal chalcogenides which contain discrete clusters of transition metal atoms via reactions between the elements have only produced mixtures of binary compounds or other ternary structures. This is probably because the desired compounds are thermodynamically unstable with respect to the known compounds in these phase diagrams. Consequently, the key to the success to their synthesis will be avoiding the formation of stable binary compounds as reaction intermediates. Towards this end, a kinetically controlled route to solid-state compounds is developed based upon the design of elemental superlattices as uniquely tailorable reactants. This approach separates the two key steps of a solid state reaction - interdi ffusion of the reactants and nucleation of the products. Since product formation is controlled by the lucleation barriers, one is able to kinetically control the product formed. The development of synthetic tools to nucleate desired structures is a significant part of the proposed research.