THEORETICAL STUDIES OF THE LOCAL STRUCTURES AND SPIN HAMILTONIAN PARAMETERS FOR THE RHOMBICNi2+CENTERS INAgX(X=Cl,Br)

Abstract

The local structures and the spin Hamiltonian parameters (zero-field splittings D and E and the anisotropic g factors gx, gyand gz) for the rhombic Ni2+centers in AgX ( X = Cl , Br ) are theoretically studied from the perturbation formulas of these parameters for a 3d8ion 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 VAgalong 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 VAgalong the [110] axis, while the ligands closest to the VAgundergo 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.