SE AND TE SUBSTITUTIONS IN YSR2CU3O7-Y AND [Y CE]2SR2CU3O9-Y/

The partial substitution of Se and Te for Cu in the ''chain'' sites in the systems YSr2Cu3O7-y and [Y/Ce]2Sr2Cu3O9-y has been examined. In t he case of Se, successful substitution was achieved with the highest-p urity samples being observed for 50% replacement of the ''chain'' Cu s ites. XPS measurements have shown that the Se is present as Se4+ in th ese phases. Although none of the samples was superconducting as prepar ed or after high-pressure oxygen annealing, superconductivity (T-c app roximate to 45 K) could be induced in YSr2Cu2.5Se0.5O7 by partial repl acement of Y by Ca. Negligible substitution of Te was observed in thes e systems.