Optical Spectra of Ni2+, Co2+, and Cu2+ in Tetrahedral Sites in Crystals

The polarized absorption spectra of Ni2+ and Co2+ in crystals of ZnO, ZnS, and CdS; Ni2+ in crystals of Cs2ZnCl4 and Cs2ZnBr4; and Cu2+ in ZnO have been measured at 4°K, 77°K, and room temperature. The spectra have been interpreted by the use of crystal field theory for the states of the (3d)n configuration acted on by a potential of predominately Td symmetry. Certain details of the spectra are accounted for by smaller contributions from fields of lower symmetry, notably a C3v potential contribution for the transition metal ions in ZnO. Crystal field, electrostatic repulsion, and spin-orbit parameters have been obtained for all these cases. An empirical correlation between the electrostatic repulsion parameter, B, for the ions in the crystals and the ligand polarizibility has been obtained. Although the configuration mixing between the states of the configurations (3d)n and (3d)n—1 (4p) has been found to give a negligible contribution to the calculated relative energies of the levels, it does partially explain the large values of the observed intensities of the transitions. The relative intensities of transitions between the spin-orbit components have been calculated by using models for both p—d mixing and σ bonding with the ligands. Neither model gives a quantitatively good account of the observed relative intensities and there is evidence, at least for the Ni2+ spectra, that the observed low energy, intense charge transfer absorptions play a predominant role in the intensity mechanisms of the d—d transitions.