Ion-size effect on ${\mathit{T}}_{\mathit{c}}$ in (${\mathit{R}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ systems (R=Nd, Eu, Gd, Dy, Y, Er, and Yb)

We observed that the superconducting-transition temperature (${\mathit{T}}_{\mathit{c}}$) of the oxides (${\mathit{R}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$)${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ (R=Nd, Eu, Gd, Dy, Y, Er, and Yb) decreases monotonically with increasing Pr concentration x. At constant x, ${\mathit{T}}_{\mathit{c}}$ decreases approximately linearly with increasing radius of the R ions. We suggest that in (${\mathit{R}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$)${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ systems the hybridization between 4f states of the Pr ion and the conduction-band states is R-ion-radius dependent.