[[abstract]]Correlation between copper weak ferromagnetism (WF) and crystal structural symmetry of the Gd2-xMxCuO4 (M=Bi or Tb; 0⩽x⩽0.5) systems is reported. Detailed powder X-ray Rietveld refinement analysis on Gd2-xMxCuO4 shows a systematic variation of oxygen distortion angle α(Cu-O-Cu) with ionic size where the lattice layer mismatch lowers the crystal symmetry to an orthorhombic O'-phase with pseudotetragonal lattice parameter a0~b0~5.508 Å. Weak ferromagnetic or canted antiferromagnetic order is the direct result of this oxygen distortion which causes a σ-transfer Cu(3d(x2-y2))-O(2pσ)-Cu(3d(x2-y2)) superexchange interaction in the CuO2 plane with a non-180° coupling angle. The small WF saturation moment ms of ~2-6×10-2μB/Cu2+ can be deduced from the copper moment μ(Cu2+) canting angle &thetas;=(π-α)/2~2-7°. Magnetic data and internal exchange field Bint estimation indicate that Cu2+ WF saturation moment ms(Cu2+) decreases with larger Bi3+ doping and increases with smaller Tb3+ doping[[fileno]]2010102010180[[department]]物理
