Crystal growth and superconducting properties of (Y,Ca)Ba2Cu3O7-delta withBaF2 additive

High-quality single crystals of YBCO:Ca up to 2x2cm in size have been grown in non-contaminating yttria or BaZrO3 crucibles from the eutectic self-flu x system with BaF2 additive. A cluster model is proposed to describe the fu nction of F- ions in the high temperature solution. The distribution coeffi cient of Ca between the solid and the liquid phases is much less than1, whi ch indicates that crystals with a Ca doping level > 0.22 are thermodynamica lly unfavourable. The preference of six-fold co-ordination for Ca substitut ed in Y-sites at low doping concentration has little effect on the lattice parameters, but it distorts the conductive Cu-O layers. At high doping conc entration, a small fraction of Ca also substitutes into Ba-sites near the C u-O chains. We propose a modified model for estimating the effective hole c oncentration which fits better the universal parabolic variation of T-C.