Effect of Th substitution on the transition temperature of Y/sub 1-x/Th/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconducting system

High temperature superconducting samples of the system Y/sub 1-x/Th/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// with x value ranging from 0.02 and up to 0.2, were prepared using the solid state reaction technique. The magnetically determined transition temperatures reflect an enhanced superconductivity at low Th concentration followed by a depression with increasing Th content. The enhanced superconductivity at low Th concentrations may be due to the slightly larger ionic radius of Th and a suggested higher hole concentration on plane oxygen sites relative to plane copper sites. On the other hand, the depression of superconductivity at higher Th concentrations could be attributed to an increased hole filling by the extra Th electron.