First-principle calculation for absorption spectrum of Cr~(4+):Ca_2MgSi_2O_7 (akermanite)

摘要

Peak energy and intensity in absorption spectrum of Cr~(4+):Ca_2MgSi_2O_7 (akermanite) were calculated by first-principles method, which we have developed. We investigated the dependence on size of cluster models with [CrO_4]~(4-) (without point charges added), [CrO_4]~(4-) (with additional point charges), and [CrCa_6Mg_2SiO_(38)]~(52-) models. All the models have low C_s symmetry at Cr~(4+) site. The best agreement with experimental spectrum was obtained in the larger [CrCa_6Mg_2SiO_(38)]~(52-) model on the relative peak splittings which originate from the low symmetry, on the spectral anisotropy, and on the relative intensity. We conclude that the covalency between ligands and the outside cations have to be considered for the better description of spectrum in system with low symmetry.