SUPERCONDUCTIVITY AND VALENCE STATES OF CE, PR AND TB IN BULK LU1-XMXBA2CU3O7-DELTA (M = CE, PR AND TB) SYSTEMS

Bulk samples of composition Lu1-xMBa2Cu3O7-delta (M=Ce, Pr and Th) are examined with respect to the formation of 1-2-3 phase and the superconducting properties. Samples with M=Pr and Tb form a superconducting 1-2-3 phase for values of x less-than-or-equal-to 0.6. We observe that in case of M=Tb, the zero-resistance transition temperature, T(c(0)), is 85-86 K and independent of x for 0.1 less-than-or-equal-to x less-than-or-equal-to 0.3. For M=Pr, a maximum T(c(0)) of 86 K is observed for x=0.1. T(c(0)) decreases with increasing x for x>0.1 and the sample with x=0.6 is observed to be semiconducting down to 12 K. With M=Ce, samples of the composition Lu1-x-yCexCaBa2Cu3O7-delta form the superconducting 1-2-3 phase for x, y=0.1 and 0.15. Samples of the compositions Lu1-x-yPrxCaBa2Cu3O7-delta with x=0.1, 0.2, 0.3 and y=0.1, have lower T(c(0))'s than samples of a corresponding Pr content without Ca. We infer from our studies that the Pr ions are in the 3+ valence state for 0.1 less-than-or-equal-to x less-than-or-equal-to 0.3, the Th ions are mainly in the 3+ valence state for 0.t less-than-or-equal-to x less-than-or-equal-to 0.3, and the Ce ions are in the 4 + state.