Studies on the spin Hamiltonian parameters and local angular distortions for the tetragonal Cu2+ centers in the ZnX (X=O and S) nanocrystals

The spin Hamiltonian parameters (g factors and hyperfine structure constants) and local structures are theoretically studied for the tetragonal Cu2+ centers in the ZnX (X=O and S) nanocrystals from the perturbation formulas of these parameters for a 3d9 ion in tetragonally distorted tetrahedra. The ligand orbital and spin–orbit coupling contributions are considered in view of strong covalency. Due to the Jahn–Teller effect, the local Cu2+‒X2− bond angles between the four equivalent impurity-ligand bonds and the four-fold axis are found to be about 2.47° and 1.68° larger than that (≈54.74°) for an ideal tetrahedron. This induces tetragonally compressed [CuX4]6− clusters on tetrahedral substitutional Zn2+ sites, different from the assignments (i.e., Cu2+ on tetragonally elongated octahedral and tetrahedral substitutional sites in the ZnO and ZnS nanocrystals, respectively) in the previous works. The calculated g factors for both systems and the parallel component of the hyperfine structure constants for the ZnS:Cu2+ nanocrystals based on the above local angular distortions are in good agreement with the observed values. The validity of the present assignments for the local structures of the Cu2+ centers is analyzed.