Effects of substitution of O by S and Cu by Fe on superconductivity in YBa2Cu3O7

The single-phase high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/S/sub x/ and YBa/sub 2/Cu/sub 3(1-//sub y//sub )/Fe/sub 3//sub y/O/sub 7/ have been prepared and studied by dc magnetometry. YBa/sub 2/Cu/sub 3/O/sub 6/S has a larger unit cell than YBa/sub 2/Cu/sub 3/O/sub 7/, and yet has the same T/sub c/; its phase transition is sharper and displays full Meissner effect. Substitution of Cu by Fe leads to a sharp decrease in T/sub c/.