On the layer sequence and morphology in solid-state displacement reactions

In displacement reactions of the type pA+B_qXqB+A_pX, two main forms of the layer sequence have been found in the reaction layer, namely, A/A_pX/B/B_qX and A/B/A_pX/B_qX. In this paper it is argued that the thermodynamics of the system, more specifically the dependence of the activity of element X on the mole ratio between the elements A and B, determines the initial layer sequence. Various morphological variants of the two basic types may develop because of the kinetics. The prediction of the layer sequence and morphology made on the basis of this theory has been verified on a number of oxidic and sulfidic systems. It is, however, pointed out that these predictions also can be applied to purely metallic ternary systems and to other types of reactions.