STABILIZATION OF NEW SUPERCONDUCTING THALLIUM CUPRATES AND OXYCARBONATES BY MOLYBDENUM

The stabilization of thallium cuprates and oxycarbonates by molybdenum has been studied. Two cuprates have been stabilized: Tl0.8Mo0.2Sr2CuO 5-delta with the ''1201'' structure which does not superconduct and th e superconductor Tl0.8Mo0.2Sr2CaCu2O7-delta with the ''1212'' structur e whose T-c increases from 40 K, when as-synthesized, to 86 K after an nealing. in a reducing atmosphere. A new superconducting oxycarbonate Tl0.8Mo0.2Sr4Cu2(CO3)O-7-delta has been synthesized, whose structure c orresponds to an intergrowth of the ''1201'' structure of Tl0.8Mo0.2Sr 2CuO5-delta with the Sr2CuO2CO3 structure. This superconductor also ex hibits a T-c ranging from 30 K, when as-synthesized, to 80 K when anne aled in a reducing atmosphere. Substitution of La for Sr in the ''1201 '' phase allows superconductivity to be induced up to 40 K, whereas su bstitution of Ba for Sr in the oxycarbonate does not significantly inf luence the superconducting properties.