DOPING STUDIES ON THE ORTHORHOMBIC TO MONOCLINIC PHASE-TRANSFORMATIONIN BI2SR2CACU2OY

Doping and magnetic studies are presented on the high-temperature phas e transformation from the superconducting orthorhombic phase of Bi2Sr2 CaCu2Oy to a non-superconducting monoclinic phase. This transformation is reversible on reoxygenation. The phase transformation is very sens itive to substitutional doping of the CuO2 planes and adjacent lattice sites, suggesting that the transformation is dominated by oxygen diff usion in the vicinity of the CuO2 planes rather than the BiO2 planes. In the monoclinic phase, the loss of oxygen results in the evolution f rom a tweed pattern, indicative of short-range order, to a unidirectio nal twinning microstructure associated with long-range structural orde ring.