MONOCLINIC PHASE-TRANSITION AND TWINNING IN BI2SR2CACU2OY SINGLE-CRYSTALS

An orthorhombic-to-monoclinic phase transition with associated (100) t winning has been observed by transmission electron microscopy in super conducting Bi2Sr2CaCu2Oy single crystals after heating to between 350 and 400-degrees-C. A model describing the monoclinic phase structure a nd the twin boundary geometrical configuration is presented. We propos e that the monoclinic phase transition results from oxygen ordering in the CuO2 Planes associated with oxygen diffusion out of the crystal a t high temperature and high vacuum. This transition is accompanied by (100) twinning to reduce the strain energy.