Superconductivity and valence states of Ce, Pr and Tb in bulk Lu<SUB>1&#8722;x</SUB>M<SUB>x</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7&#8722;&#948;</SUB> (M = Ce, Pr and Tb) systems

Bulk samples of composition Lu1&#8722;xMxBa2Cu3O7&#8722;&#948; (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&#8804;0.6. We observe that in case of M = Tb, the zero-resistance transition temperature, Tc(0), is 85-86 K and independent of x for 0.1 &#8804;x &#8804;0.3. For M = Pr, a maximum Tc(0) of 86 K is observed for x=0.1. Tc(0) decreases with increasing x for x &gt; 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&#8722;x&#8722;yCexCayBa2Cu3 O7&#8722;&#948; form the superconducting 1-2-3 phase for x, Y=0.1 and 0.15. Samples of the compositions Lu1&#8722;x&#8722;yPrxCayBa2Cu3 O7&#8722;&#948; with x=0.1, 0.2, 0.3 and y=0.1, have lower Tc(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&#8804;x&#8804;0.3, the Tb ions are mainly in the 3+ valence state for 0.1&#8804;x&#8804;0.3, and the Ce ions are in the 4+ state