THEORETICAL STUDIES OF THE LOCAL STRUCTURES AND SPIN HAMILTONIAN PARAMETERS FOR THE RHOMBIC Ni2+ CENTERS IN AgX (X = Cl, Br).

The local structures and the spin Hamiltonian parameters (zero-field splittings D and E and the anisotropic g factors gx, gy and gz) for the rhombic Ni2+ centers in AgX (X = Cl, Br) are theoretically studied from the perturbation formulas of these parameters for a 3d8 ion under rhombically distorted octahedra. In these formulas, the ligand p- and s-orbital and spin-orbit coupling contributions are included on the basis of the cluster approach. The rhombic centers are attributed to the impurity Ni2+ associated with one nearest-neighbor silver vacancy VAg along the [110] direction as charge compensation. Based on the calculations, Ni2+ is found to suffer an off-center displacement 0.092 Å (or 0.335 Å) for AgCl (or AgBr) towards the VAg along the [110] axis, while the ligands closest to the VAg undergo a small shift 0.065 Å (or 0.006 Å) away from (or towards) the VAg. The theoretical spin Hamiltonian parameters based on the above defect structures show good agreement with the experimental data. [ABSTRACT FROM AUTHOR]