Theoretical Studies on the Local Structures and EPR‐ <i>g</i> Factors for the Orthorhombic Cu ²⁺ Centers in GdBa ₂ Cu ₃ O _7‐Δ and La _0.5 Gd _0.5 Ba ₂ Cu ₃ O <sub>y

The local structures and the electron paramagnetic resonance (EPR) g factors of Cu²⁺ centers in GdBa₂Cu₃O_7-Δ and La_0.5Gd_0.5Ba₂Cu₃O_y are theoretically studied by using the perturbation formulas of the g factors for an orthorhombically elongated octahedral 3d⁹ cluster. In these formulas, the crystal-field parameters (CFPs) are determined from the superposition model and the local distortions due to the Jahn-Teller (JT) effect. Based on the calculations, the Cu-O bond lengths are found to experience the axial elongations δz (≈0.052 and 0.055 Å) along the c-axis and the planar bond length variations δr (≈0.061 and 0.128 Å) in the perpendicular (ab) plane for the orthorhombic Cu²⁺ centers in GdBa₂Cu₃O_7-Δ and La_0.5Gd_0.5Ba₂Cu₃O_y, respectively. The calculated g factors are in good agreement with the experimental data. The local structures of both Cu²⁺ centers are discussed.