Depression of T<SUB>c</SUB> with rare earth substitution for La in La<SUB>2-x</SUB>Sr<SUB>x</SUB>CuO<SUB>4</SUB> superconductors

Phases of the type La2-x-yRxSryCuO4-z, whereR is Pr, Nd, Sm, Eu, or Gd, have been prepared for various values ofx andy. All phases prepared had the La2CuO4 structure. The general trend for fixedy is that Tc decreases as the four coplanar Cu---O distances decrease. This unexpected correlation between Tc and Cu-O distance is explained by our finding that the CuIII concentration also decreases as these R cations are substituted for La. The amount of R that can be substituted for La increases as R becomes larger. For R =Pr, the x value reaches about 0.8. Such phases, for example, La1.05Pr0.80 Sr0.15CuO4, do not show a depressed Tc, indicating that Pr does not always adversely affect superconductivity in copper oxide based phases as it does in the YBa2Cu3O7 type structure