Study on the covalence effect of the transition-metal in solids and its application for the electron states of ZnS:Co2+ crystal

In the investigation of optical and magnetic properties of the semiconductors containing transition-metal ions, the one-electron orbital cannot be treated as a pure d-orbital because of a strong covalence. In the energy matrix, the covalence could be described by two covalent factors (Nt and Ne) associated with the t2g and eg orbits. This paper studied the contribution of the covalent factors for d7 electric and d3 hole systems on the basis of a semi-SCF model and the complete energy matrix diagonalization procedure (CDP). Both the matrices of electrostatic interaction and the crystal field potential are described in generalized forms. In these forms, d7 electron system cannot be respected as d3 hole system because their energy matrices are different. Nt and Ne are not identical in the molecular orbit (MO) calculation, but a numerical calculation shows that the difference in numerical values should be small, because Racah electrostatic parameter A plays giant role when the covalence effect considered, which means that the conventional B, C and Δ scheme of crystal theory is a good approximation. These conclusions are then extended to the application of ZnS:Co2+, the energy levels and g-factor are explained uniformly.