Comparative first-principles study of the Ni²⁺ absorption spectra and covalence effects in isostructural crystals NiCl₂, NiBr₂ and NiI₂

Systematic ab initio calculations of the ground-state absorption spectra and covalence effects in three isostructural anisotropic crystals with layered structure (NiCl₂, NiBr₂ and NiI₂) are presented. The recently developed first-principles approach to the analysis of the absorption spectra based on the discrete variational multi-electron method (K. Ogasawara et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, energies of the Ni²⁺-ion multiplets and absorption spectra of all above-mentioned crystals were calculated, assigned and compared with experimental spectra. By performing analysis of the molecular orbital population, it was shown that the degree of covalence of the chemical bond between Ni²⁺ ion and ligands is in the following order: NiI₂>NiBr₂>NiCl₂.