The possibility of negative substitution (x) in melt-processed Gd1+xBa2−xCu3O7−δ GdBCO bulk superconductors

We have reported previously the successful fabrication of single grain Gd–Ba–Cu–O (GdBCO) bulk superconductors in air by top seeded melt growth (TSMG) using a generic Mg–Nd–Ba–Cu–O (Mg–NdBCO) seed crystal. Here, we report a new processing route that enables the fabrication of bulk GdBCO single grains in air using precursors with higher Ba content than has been achieved in previous studies. In particular, the composition of the superconducting matrix of single grains prepared with high, but varying, levels of Ba is analyzed by EPMA. The degree of substitution Ba for Gd (corresponding to a negative value of x in the composition formulation), in GdBCO is proposed for the first time based on the superconducting, compositional, chemical and structural properties of the Gd-123ss (Gd 1+ x Ba 2− x Cu 3 O 7− δ ) phase matrix observed this study.