SPIRAL GROWTH MECHANISMS IN PARTIALLY MELTED BULK YBA2CU3O7-DELTA

Large domains with platelets almost parallel to each other were obtain ed in bulk YBa2Cu3O7-delta by a single-step partial melting procedure. The mechanisms of nucleation and growth of platelets are discussed. T he nucleation of peritectic material from the liquid phase is favoured by heterogeneities in the melt. Experimental evidence of spiral growt h of the nuclei in the [001] direction is given. Furthermore, structur es of growth, which could also be an indication of spiral growth in th e [010]/[100] directions, are shown. The final morphology of the domai ns can be explained on the basis of the periodic bond chain (PBC) theo ry if the growth rates of flat (F) faces of the platelets are dominate d by kinetic coefficients which differ between them. The morphology of the as-grown (001) surface is explained in the framework of the PBC t heory with the shape of the steps of macrospirals governed by the tran sition from roughness to smoothness of the liquid-solid interface. An account of large step heights is given by the model of giant screw dis locations caused by an impurity-induced lattice-constant gradient. Eve n higher step heights are correlated to the presence of obstacles and lack of liquid phase.