Ionic-size effect on the structure and T-c, of T-'-(R(1-x)R'(x))(1.85)Ce0.15CuO4(R=Pr,Nd,Sm and Eu; R'=Gd and Y)

(R(1-x)R'(x))(1.85)Ce0.15CuO4 (R = Pr, Nd, Sm and Eu; R' = Gd and Y) have been investigated by means of X-ray diffraction, electrical-resistivity, and magnetic-susceptibility measurements. Lattice parameters a, c, and the unit-cell volume V decrease with increasing Gd or Y concentration x, and the decreasing rates with Y are larger than those with Gd. It is found that the critical concentration x(c), at which the superconductivity disappears, depends on the ionic size of R, and the T-c suppression rate for each system with Y doping is larger than that with Gd doping, Thus the effect of a small ionic radius for Y plays a much more important role than the large magnetic moment for Gd on the suppression of superconductivity in (R(1-x)R'(x))(1.85)Ce0.15CuO4. From the observations of the deviation of the lattice parameters in R(1.85)Ce(0.15)CuO(4) and the rapid increase of \dln T-c/dx\ in (Eu1-xYx)(1.85)Ce0.15CuO4, the structural distortion boundary is estimated to lie between Eu1.85Ce0.15CuO4 and Gd1.85Ce0.15CuO4. This provides clear evidence that the absence of superconductivity for Gd1.85Ce0.15CuO4 is due to weak ferromagnetism in the Cu-O planes resulting from the too small ionic radius of Gd, which induces a lattice distortion in the T' structure.