Spin Hamiltonian parameters and local structures for tetragonal and orthorhombic Ir2+ centers in AgCl

The perturbation formulae of the spin Hamiltonian parameters ( the anisotropic g factors, hyperfine structure constants and superhyperfine parameters) are established for a 5d(7) ion in an orthorhombically elongated octahedron based on the cluster approach. These formulae are applied to the theoretical studies of the EPR spectra and the local structures for the tetragonal and orthorhombic Ir2+ centers in AgCl. For the tetragonal Ir2+ center, the uncompensated substitutional [IrCl6](4) cluster is found to experience a relative elongation of about 0.08 angstrom along the C-4 axis due to the Jahn-Teller effect. For the orthorhombic center, the ligand octahedron also suffers Jahn-Teller elongation (by about 0.08 angstrom) along the [001] (or Z) axis. Meanwhile, the ligand Cl intervening in the impurity Ir2+ and the next nearest neighbor silver vacancy V-Ag along the [100] (or X) axis may undergo an inward displacement of 0.004 angstrom towards the center of the octahedron due to electrostatic repulsion of the V-Ag. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with experimental data for both centers.