Investigation of relations between absorption band positions and crystalline environment in Pb2+-doped alkali halides

Abstract

The relationships between sp energy levels (A, B and C bands) as well as the charge transfer band (D band) of Pb²⁺-doped alkali halides and the crystalline environment were thoroughly investigated by means of dielectric theory of chemical bonds 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 major factors influencing the positions of A, B, C and D bands of Pb²⁺. Our model has successfully built links between the E_A, E_B, E_C and E_D of Pb²⁺ and the environmental factor h. Results indicated that the energies of sp levels and the charge transfer band of Pb²⁺ all decrease with increase of h. The h has a linear relationship with the sp energy levels, and an exponential relationship with the charge transfer band. The model calculation results are in good agreement with experimental data. The current model can serve as a prediction tool and can be applied to assign and reassign the A, B, C and D band positions of Pb²⁺. The model predicts the positions of B, C and D bands of NaF:Pb²⁺ at 7.516, 8.688 and 12.796 eV, respectively; the D band of CsCl:Pb²⁺, CsBr:Pb²⁺ and CsI:Pb²⁺ at 6.633, 6.389 and 5.275 eV, respectively. We reassign the C band of Pb²⁺ in NaBr as 5.276 eV but not the reported 5.636 eV, which is more reasonable to be ascribed to the charge transfer band.