Comparative crystal field study of Ni²⁺ energy levels and crystal field effects in CsCdBr₃ and CsMgBr₃ crystals

The exchange charge model of crystal field theory has been used to analyze the ground state absorption of Ni²⁺ ions in CsCdBr₃ and CsMgBr₃ crystals. The parameters of crystal field were calculated from the available crystal structure data. The obtained Ni²⁺ energy level schemes were compared with the experimental absorption spectra; good agreement between the calculated and experimental data was demonstrated. Dependencies of the crystal field strength 10Dq on the "impurity ion - ligand" distance of the nearest Br^- ions from Ni²⁺ ions were derived for both crystals and used to estimate the Stokes shift values for these crystals; all calculated results are close to the experimental data. The "critical" Ni-Br distances leading to the Ni²⁺ energy levels crossing were estimated; these results can be used for the analysis of the external hydrostatic pressure effects on the optical properties of the studied crystals.