The defect structures and the electron paramagnetic resonance (EPR) parameters zero-field
splitting and the g factors for the tetragonal and cubic Ni2+ centers in AgX (X=Cl, Br) are theoretically
studied from the perturbation formulas of these parameters for a 3d8 ion in tetragonal and cubic
octahedra. In these formulas, the ligand s-orbital contributions, which were usually neglected in the
previous studies, are taken into account based on the cluster approach. The ligand X− intervening in
the Ni2+ and the next nearest neighbour Ag+ vacancy VAg is suggested to suffer a small inward
displacement (≈ 0.11 or 0.15Å for AgCl or AgBr, respectively) towards Ni2+ along the [100] (or C4)
axis. The calculated EPR parameters based on the above defect structures show reasonable agreement
with the experimental data.