The defect structure and spin Hamiltonian parameters ([Formula: see text] factors [Formula: see text], [Formula: see text] and the hyperfine structure constants [Formula: see text] and [Formula: see text]) for the tetragonal [Formula: see text] in [Formula: see text] ceramics are theoretically studied from the perturbation formulas of these parameters for a [Formula: see text] ion in a tetragonally elongated octahedron. The impurity [Formula: see text] center exhibits the larger relative tetragonal elongation ratio [Formula: see text] than that [Formula: see text] of the host [Formula: see text] site due to the Jahn–Teller effect. The relationship between the [Formula: see text] anisotropy [Formula: see text] [Formula: see text] and the local tetragonal distortion is analyzed. The present studies would be helpful to the understandings of delocalization of the [Formula: see text] [Formula: see text] orbitals under tetragonal elongation distortion and its role in superconductivity of oxygen cuprates.

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The anisotropic [Formula: see text] factors of the deoxygenated YBaCuO (Y123) are theoretically studied using the perturbation formulas of the [Formula: see text] factors for a tetragonally elongated octahedral [Formula: see text] cluster. The ligand orbital and spin-orbit coupling contributions are included from the cluster approach in view of covalency. The calculated [Formula: see text] factors show good agreement with the experimental data. The anisotropy of the [Formula: see text] factors is analyzed by considering the local tetragonal elongation distortion around this five-fold coordinated [Formula: see text] site in the deoxygenated Y123 system.