Dislocation-assisted-diffusion model and the mechanism for bismuth cuprate 2212-to-2223 transformation

Recent transmission electron microscopy (TEM) experiments have shown that the transformation of the Bi cuprate 2212 phase to 2223 phase consists of random intercalation and fast growth of Ca/CuO2 planes in the Bi-2212 matrix. This contridicts with the prediction of the conventional nucleation and growth theory. A model is proposed which considers the cylindrical void created by the edge dislocation due to the insertion of a Ca/CuO2 plane to be a channel for fast ion diffusion. A layer-rigidity model is used to estimate the size of the void for Bi-2312/Bi-2223. The calculated void size for Bi-2212;Bi-2223 compounds is consistent with the existence of fast ion diffusion along the ab-plane. The experimentally observed time dependence of the volume fraction of Bi-2223 during the transformation is found to be consistent with a one-dimensional diffusion-controlled transformation mechanism, as predicted by our model. (C) 1997 Elsevier Science S.A.