Superconductivity and valence states of Ce, Pr and Tb in bulk Lu_1−xM_xBa₂Cu₃O_7−δ (M = Ce, Pr and Tb) systems

Abstract

Bulk samples of composition Lu_1−xM_xBa₂Cu₃O_7−δ (M=Ce, Pr and Tb) 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≤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 ≤x ≤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 Lu_1−x−yCe_xCa_yBa₂Cu₃ O_7−δ form the superconducting 1-2-3 phase for x, Y=0.1 and 0.15. Samples of the compositions Lu_1−x−yPr_xCa_yBa₂Cu₃ O_7−δ 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≤x≤0.3, the Tb ions are mainly in the 3+ valence state for 0.1≤x≤0.3, and the Ce ions are in the 4+ state.