BO33- SUBSTITUTIONS IN PHASES RELATED TO YBA2CU3O7 - SUPERCONDUCTING PROPERTIES OF (Y, CA) (BA, SR)2CU3-Z(BO3)ZOY

The effects of partially replacing Cu by borate anions in the system Y Sr2-xBaxCu3-z(BO3)zOy (YSr2-xBaCu3-zBzO7-delta) have been examined. Th e optimum substitution level to give single-phase samples was found to be z almost-equal-to 0.5. Borate-substituted samples of this composit ion were found to be non-superconducting as prepared, but superconduct ivity could be induced by the partial replacement of Y by Ca (for samp les with 0.5 less-than-or-equal-to x less-than-or-equal-to 1.5), or by high oxygen pressure treatment (for 1.0 less-than-or-equal-to x less- than-or-equal-to 1.5). The highest T(c) onset recorded for a single-ph ase sample was 55 K for Y0.85Ca0.15Ba1.5Sr0.5Cu2.5B0.5O7 annealed in h igh pressure oxygen. The unit cell symmetry changes from orthorhombic for the Ba-free phase (x=0) to tetragonal at higher Ba contents, which appears to be associated with a randomisation of the orientations of the borate groups.