DISLOCATION-ASSISTED-DIFFUSION MODEL AND THE MECHANISM FOR BISMUTH CUPRATE 2212-TO-2223 TRANSFORMATION

Recent transmission electron microscopy (TEM) experiments have shown t hat the transformation of the Bi cuprate 2212 phase to 2223 phase cons ists of random intercalation and fast growth of Ca/CuO2 planes in the Bi-2212 matrix. This contridicts with the prediction of the convention al nucleation and growth theory. A model is proposed which considers t he cylindrical void created by the edge dislocation due to the inserti on 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 consi stent 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-di mensional diffusion-controlled transformation mechanism, as predicted by our model. (C) 1997 Elsevier Science S.A.