The Use of Superlattice Reactants in the Synthesis of Ternary CU-NB-SE Compounds
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these compounds are to be synthesized, it will require a low-temperature, kinetically controlled synthesis that is capable of avoiding the formation of the stable binaries. Recently, Fister and co-workers have demonstrated the ability to prepare the known Chevrel compound CuxMo6Se8 16 and Franzen and co-workers have prepared PbMo 6 S817 from amorphous intermediates without the formation of binary reaction intermediates. This synthesis approach is based on the formation of an amorphous reaction intermediate by interdiffusing thin film elemental multilayersI1-1 5 or by co-deposition of the elements 17 . The amorphous reaction intermediate is the key to this approach since the compound which forms from this intermediate depends upon nucleation energetics rather than the absolute thermodynamic stability of the final product. Thus the amorphous intermediate can be used as precursor to directly crystallize a ternary compound while avoiding the more stable binary phases. The general synthetic approach used in our laboratory is to study each of the binary systems to understand the dependence of the reaction mechanism on layer thickness. Typically there is a critical thickness below which elements interdiffuse to form an amorphous intermediate before crystallizing. Ternary multilayers are then prepared with the layer thicknesses less than the critical thicknesses determined from the binary studies. The multilayers are then annealed to produce the amorphous precursor which is then used in attempts to crystallize the desired compound. In this paper we report the results from our attempts to synthesize a Cu-Nb-Se analog of the Chevrel phases using this approach. A summary of results on the binary Nb-Se and Cu-Se systems is presented. These results were used to prepare amorphous ternary intermediates and known ternary compounds without the formation of crystalline binary compounds as reaction intermediates. EXPERIMENTAL PROCEDURES The multilayer samples were prepared in a high-vacuum evaporation system which has been described in detail elsewhere. Briefly, the elements were sequentially evaporated under high vacuum (approx. 5x10- 7 torr) under the control of a personal computer. A Knudsen cell was used as the selenium source. Copper and niobium were evaporated using electron beam evaporators. Each source was independently monitored with quartz crystal thickness monitors. The deposition of each layer was controlled to the nearest angstrom and all elements were deposited at 0.5 A/sec. The stoichiometries of samples prepared over the course of this study were found to be repeatable to within about 5%. Photoresist coated silicon wafers were used as substrates. The samples were removed from the substrates by dissolving the resist in acetone and then collecting and vacuum drying the samples. The total mass of each sample was approximately 2-3 mg. The stoichiometry of the binary samples were determined by thermal-gravimetric analysis. Approximately 0.5 nmg was oxidized to form CuO or Nb 2 0 5 and the selenium content calculated from
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