Molecular orbital model for antimony and bismuth luminescent centers in antimonates M(II) Sb2O6 (M=Ca,Sr,Ba)

Abstract

A detailed semiempirical molecular orbital calculation is presented for computing the energy levels of various luminescent centers in the antimonates. In order to interpret the excitation spectrum of the nondoped compounds, the MO calculations are carried out on two clusters: (SbO₆)⁷⁻ and (SbO₆)⁹⁻. In the first one, the central metal is Sb⁵⁺ with a point symmetry D₃; in the second one the metallic ion is Sb³⁺, occupying the M(II) site with a point symmetry D_3d. The results for (SbO₆)⁷⁻ do not agree at all with experiment; therefore, the possibility of a charge transfer process from the ligands to the Sb⁵⁺ ion is eliminated. However, good agreement with experiment is found for the (SbO₆)⁹⁻ calculations, supporting the idea that the electronic transitions may be assigned to centers Sb³⁺, produced by an oxido‐reduction process, Sb⁵⁺?Sb³⁺, during the preparation of the compounds and placed at M(II) sites. The results concerning the pseudomolecule (BiO₆)⁹⁻ are in good agreement with the experiment in the case of antimonates doped by bismuth, permitting one to assign the observed excitation bands of bismuth to the low‐lying excited triplet states.