The effect of layer thickness and composition on the kinetics of solid state reactions in the niobium-selenium system studied using superlattice reactants

The ability to form an amorphous reaction intermediate by the low temperature interdiffusion of a modulated elemental reactant is shown to be a function of the overall composition as well as elemental layer thicknesses in the niobium-selenium system. For niobium-rich reactants, an amorphous reaction intermediate was observed to form upon low temperature annealing of reactants with modulation thicknesses less than 60 Angstrom. Further annealing of the amorphous intermediates led to the crystallization of Nb2Se, Nb5Se4 or Nb3Se4 depending upon the overall composition of the amorphous intermediate. Modulated elemental reactants with overall compositions containing more than two-thirds selenium were found to heterogeneously nucleate NbSe2 at the reacting interfaces. The formation of the thermodynamically expected compounds Nb2Se3, NbSe3, and Nb2Se9 at their respective compositions required extended high temperature annealing to react the dichalcogenide with the remaining elemental reactants. A striking difference between the evolution of the low angle diffraction patterns in these two composition regimes suggests the differences in the reaction kinetics result from a composition dependence of the diffusion coefficients.