Making Amorphous and Crystalline Alloys by Solid State Interdiffusion

This method is a new synthetic approach in which thin (15-50 A) amorphous elemental layers are sequentially deposited to create a unique initial reactant.  The 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 the new synthetic approach, the initial layered composite is diffused at low temperatures to 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 x-ray diffraction.

Composition-directed crystallization of iron silicides from homogeneous amorphous alloy intermediates is a first example of the new synthetic method.  The resultant iron silicides all crystallize below 550 degrees Centigrade, including FeSi, which is metastable with respect to a mixture of FeSi and FeSi below 825 degrees Centigrade.