Abstract
Metal-insulator transition in the Gd-doped (R 1-y Gd y ) 1.85 Ce 0.15 CuO 4-δ systems occurred around y ∼ 0.1 for R = Eu and y ∼ 0.65 for R = Nd with a simultaneous structural transition from the tetragonal T′-phase (space group I4/mmm) to an orthorhombic O′-phase (Acam, a 0 ≤ b 0 ∼ √2a t ). The structural distortion is due to the lattice mismatch between smaller (R,Gd,Ce) 2 O 2 layer and CuO 2 plane which results in an oxygen distortion in the otherwise perfect CuO 2 square plane. The phase diagram indicates that the superconductivity can only occur in the metallic T′-phase cuprates where the metallic state is controlled by a delicate balance between oxygen reduction and structural stability. The Cu 2+ weak-ferromagnetic/canted-antiferromagnetic (WF/CAF) order observed in the insulating region is the direct result of non-180° α(Cu-O-Cu) canting angle of the O′-phase. © 2000 Elsevier Science B.V. All rights reserved.