Structural, electronic and optical properties of pure and Ni 2+-doped CdI 2 layered crystals as explored by ab initio and crystal field calculations

Abstract

Influence of impurity Ni 2+ ions on optical absorption spectra of layered CdI 2 single crystals has been considered for localized level of doping. Optical properties of CdI 2:Ni 2+ crystals were modeled using two independent approaches: (i) DFT-based ab initio calculations and (ii) semi-empirical crystal field theory. The former method allowed for locating the Ni 2+ 3d states with respect to the host’s band structure, providing a link between the properties of impurity and host itself. The latter method allowed for calculations of crystal field splitting of the Ni 2+ LS terms, giving an opportunity to assign the main bands in the absorption spectrum of the doped crystal. To increase accuracy in calculating the point charge contribution to the crystal field parameters (CFP), contributions of all crystal lattice ions located at a distance of up to 72.999 Å from the Ni ion were included into the crystal lattice sums. The crystal field Hamiltonian was diagonalized in the space of 25 wave functions of the spin-triplet terms 3F, 3P and the spin-singlet terms 1S, 1D, 1G of the 3d 8 electron configuration of Ni 2+ ion. Additional calculations of the band structure and optical functions were performed to reveal the structure of the energy bands, their role in the formation of optical properties of this system in the overlapping impurity-ligand effects. Electron density distribution in the space between atoms before and after doping was compared; hybridization of the Ni 3d states with iodine 5p states was demonstrated. The role of the crystal anisotropy in the observed effects is discussed.