Variation of crystal symmetry, superconductivity and weak ferromagnetism in (R1-yGdy)(1.85)Ce0.15CuO4-delta (R = Nd, Eu) cuprates

Metal-insulator transition in the Gd-doped (R1-yGdy)(1.85)Ce0.15CUO4-delta systems occurred around y similar to 0.1 for R = Eu and y similar to 0.65 f or R = Nd with a simultaneous structural transition from the tetragonal T'- phase (space group 14/mmm) to an orthorhombic O'-phase (Acam, a(o) less tha n or equal to b(o) similar to root 2a(t)). The structural distortion is due to the lattice mismatch between smaller (R,Gd,Ce)(2)O-2 layer and CuO2 pla ne which results in an oxygen distortion in the otherwise perfect CuO2 squa re plane. The phase diagram indicates that the superconductivity can only o ccur in the metallic T'-phase cuprates where the metallic state is controll ed by a delicate balance between oxygen reduction and structural stability. The Cu2+ weak-ferromagnetic/canted-antiferromagnetic (WF/CAF) order observ ed in the insulating region is the direct result of non-180 degrees x(Cu-O- Cu) canting angle of the O'-phase. (C) 2000 Elsevier Science B.V. All right s reserved.