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.