Depression of T_c with rare earth substitution for La in La_2-xSr_xCuO₄ superconductors

Abstract

Phases of the type La_2-x-yR_xSr_yCuO_4-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 T_c decreases as the four coplanar Cu---O distances decrease. This unexpected correlation between T_c and Cu-O distance is explained by our finding that the Cu^III 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, La_1.05Pr_0.80 Sr_0.15CuO₄, do not show a depressed T_c, indicating that Pr does not always adversely affect superconductivity in copper oxide based phases as it does in the YBa₂Cu₃O₇ type structure.