Crystal growth and superconducting properties of (Y,Ca)Ba 2Cu 3O 7−δ with BaF 2 additive

Abstract

High-quality single crystals of YBCO:Ca up to 2×2 cm in size have been grown in non-contaminating yttria or BaZrO 3 crucibles from the eutectic self-flux system with BaF 2 additive. A cluster model is proposed to describe the function of F − ions in the high temperature solution. The distribution coeffecient of Ca between the solid and the liquid phases is ⪡1, which indicates that crystals with a Ca doping level > 0.22 are thermodynamically unfavourable. The preference of six-fold co-ordination for Ca substituted 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 concentration, a small fraction of Ca also substitutes into Ba-sites near the CuO chains. We propose a modified model for estimating the effective hole concentration which fits better the universal parabolic variation of T C .