EFFECT OF Y-CA SUBSTITUTION UPON SUPERCONDUCTIVITY IN THE OXIDE YBA2CU3-XCOXO7-DELTA

The coupled substitution Y-Ca / Cu-Co in YBa2Cu3O7 has been studied. The oxides Y1-yCayBa2Cu3-xCoxO7-delta have been synthesized for 0 less-than-or-equal-to x less-than-or-equal-to 0.45 and 0 less-than-or-equal-to y less-than-or-equal-to 0.30. They were found tetragonal for x great-than-or-equal-to 0.09. The oxygen stoichiometry was determined by TGA showing delta-values ranging from 6.78 to 7.08. The study of the superconducting properties of these phases shows that for a fixed cobalt content T(c) increases dramatically as the calcium content increases, leading for instance for the oxide Y0.64Ca0.36Ba2Cu2.64Co0.36O6.87 to a T(c) of 82 K, where as the non-substituted phase YBa2Cu2.64Co0.36O7-delta does not superconduct. As a result, the "compensated oxide" Y1-xCaxBa2Cu3-xCoxO7-delta, exhibits an almost x-independent T(c) ranging from 86 to 82 K for 0.15 less-than-or-equal-to x less-than-or-equal-to 0.36. A correlation is observed between the T(c) value and the c/a ratio. This behaviour is interpreted in terms of mobile holes concentration related to bond length changes induced by calcium, assuming that the cobalt is trivalent and sits in Cu1 sites in agreement with previous neutron diffraction studies of YBa2Cu3-xCoxO7-delta.