NEW SYNTHETIC APPROACH TO EXTENDED SOLIDS - SELECTIVE SYNTHESIS OF IRON SILICIDES VIA THE AMORPHOUS STATE

A new synthetic approach in which thin (15-50 angstrom) amorphous elemental layers are sequentially deposited to create a unique initial reactant is described. This new approach overcomes the limitations of traditional, diffusion-limited, solid-state synthetic methods, which offer no control of the reaction pathway and therefore no control over which intermediates are formed. In our synthetic approach, the initial layered composite is diffused at low temperaturesto produce a homogeneous amorphous alloy; nucleation is then the rate-limiting step in the formation of a crystalline material. The layered nature of the starting reactant permits the reaction to be followed in a quantitative manner with use of X-ray diffraction. The composition-directed crystallization of iron silicides from homogeneous amorphous alloy intermediates is described as a first example of our new synthetic method. The resultant iron silicides all crystallized below 550-degrees-C, including Fe5Si3, which is metastable with respect to a mixture of FeSi and FeSi3 below 825-degrees-C.