Spiral growth mechanisms in partially melted bulk YBa2Cu3O7-δ

Large domains with platelets almost parallel to each other were obtained in bulk YBa2Cu3O7−δ by a single-step partial melting procedure. The mechanisms of nucleation and growth of platelets are discussed. The nucleation of peritectic material from the liquid phase is favoured by heterogeneities in the melt. Experimental evidence of spiral growth of the nuclei in the [0 0 1] direction is given. Furthermore, structures of growth, which could also be an indication of spiral growth in the [0 1 0]/[1 0 0] directions, are shown. The final morphology of the domains can be explained on the basis of the periodic bond chain (PBC) theory if the growth rates of flat (F) faces of the platelets are dominated by kinetic coefficients which differ between them. The morphology of the as-grown (0 0 1) surface is explained in the framework of the PBC theory with the shape of the steps of macrospirals governed by the transition from roughness to smoothness of the liquid-solid interface. An account of large step heights is given by the model of giant screw dislocations caused by an impurity-induced lattice-constant gradient. Even higher step heights are correlated to the presence of obstacles and lack of liquid phase.