LARGE, FLUX-FREE Y1BA2CU3O7-DELTA SINGLE-CRYSTALS BY IMPROVED CUO-BAO FLUX GROWTH

Y1BaCu3O7-delta single crystals were grown using a modified CuO-BaO self flux method. The nominal composition and the temperature program were optimized to decrease soaking time and to obtain crystals of a cubic habit. The use of BaO instead of BaCO3 enhances the melting of the powder; so did the use of an Ar atmosphere. Melting, growth and cooling was done with different vertical temperature gradients in a specially designed chamber furnace, allowing strong convection during melting and nucleation at the bottom of the crucible, respectively. After in situ decanting of the residual flux, the free standing crystals were pre-oxygenized while slowly cooling to room temperature in oxygen. The crystals had a nearly cubic shape with a size of up to 4 x 3 x 1.5 mm3 and reached a T(c) of 91.7 K with a transition width of less than 0.5 K after a two month oxygen anneal.