Investigations on the local angular distortions and the spin-Hamiltonian parameters for Ni in sulfides

The local angular distortions and the spin-Hamiltonian parameters (the g factors and the hyperfine parameters) for Ni⁺ in ABS₂ (ACu, Ag; BAl, Ga) ternary sulfides are theoretically investigated from the perturbation formulas of these parameters for 3d⁹ ions in a tetragonally distorted tetrahedron. In view of the strong covalency of such systems, the ligand orbital and spin–orbit coupling contributions are taken into account using the cluster approach. The local impurity-ligand bond angles in the Ni⁺ centers are found to be about 1.4–4.5° smaller than those of the host monovalent A sites in the pure crystals, due to size mismatching substitution. As a result, the ligand tetrahedra exhibit slight elongation in CuBS₂:Ni⁺ and slight compression in AgGaS₂:Ni⁺. The calculated spin-Hamiltonian parameters, optical transitions and the relative intensity ratios show reasonable agreement with the experimental data.