Abstract
(R 1-x R′ x ) 1.85 Ce 0.15 CuO 4 (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 Ce 0.15 CuO 4 . 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 Tc/d x| in (Eu 1-x Y x ) 1.85 Ce 0.15 CuO 4 , the structural distortion boundary is estimated to lie between Eu 1.85 Ce 0.15 CuO 4 and Gd 1.85 Ce 0.15 CuO 4 . This provides clear evidence that the absence of superconductivity for Gd 1.85 Ce 0.15 CuO 4 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.