Chemical trends of electronic and optical properties of ns2 ions in halides
Heavy 6p and 5p ions in groups IIIB, IVB, and VB (Tl, Pb, Bi, In, Sn, Sb) are multivalent ions, which act as electron and hole traps and radiative recombination centers in many wide band gap materials. In this paper, Tl+ as a prototypical ns2 ion (ns2 ions here refer to 6p and 5p ions with outer electronic configurations of ns2) is studied as a luminescent center in alkali halides. Density functional calculations reveal the chemical trend that determines the luminescence mechanism in ns2-ion activated alkali halides. The activator–halogen hybridization strength and the ionicity of the host material strongly affect the positions of the activator levels relative to the valence and conduction band edges. This determines whether the radiative recombination occurs within the activator ion or involves the hole polaron, or the Vk center. Strategies for exploring different combinations of host materials and activators for desired luminescence mechanisms are discussed. The insight obtained in this work will help the search and the design of more efficient scintillators and phosphors.