The exchange charge model of crystal field theory has been used to analyze systematically the ground state absorption spectra of octahedrally coordinated Ni2+ ion in isostructural nickel halide crystals NiCl2, NiBr2, and NiI2. The parameters of the crystal field acting on the Ni2+ ion are calculated from the available crystal structure data. The obtained energy level schemes are compared with experimental absorption spectra; a good agreement with experimental data is demonstrated. Dependencies of the crystal field invariants and covalence effects on the type of ligands are considered. It is shown numerically that the overlap effects between Ni2+ ion and ligands increase with an increase of the ligands' atomic number resulting in the following order of the degree of covalence: NiI2>NiBr2>NiCl2. ?? 2005 Elsevier B.V. All rights reserved.
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