Systematic study of the relationships between energy levels of Tl(1), Tl centers and the crystalline environment

In this paper, we present the first systematic study of the relationships between energy levels of Tl⁰(1) and Tl²⁺ centers in alkali halides and the crystalline environment by using dielectric theory of chemical bond for complex crystals. It is found that the coordination number of the central ion, the bond volume polarizability, and the fractional covalence of the chemical bond between the central ion and the nearest anion are the three determinative parameters for the absorption band energy. Four empirical formulas are proposed. The calculated results are in good agreement with the experimental values. The current model can serve as a prediction tool and can be applied to assign the absorption band energy of Tl⁰(1) and Tl²⁺ centers.