First principles study on low valence states photoluminescence in Bi-doped M₂B₅O₉Cl crystals

The multi-valence states of bismuth ions diversify the photoluminescence in bismuth doped materials and it is of great significance to analyze the main characteristic properties of different valence states. First principles calculations are carried out for Bi³⁺, Bi²⁺, Bi¹⁺ and Bi⁰ valence states of substitutional and interstitial bismuth defects. The formation energies and charge-state transition levels results both confirm that all these valence states can be stable under appropriate chemical circumstances. The excitation and emission energies are calculated for various transition processes, and compared to experimental results. Furthermore, the calculations clearly show the trend of red shift of the emission energy with lowering the valence state, which is of significance for understanding the luminescent properties of low valence states of bismuth in M₂B₅O₉Cl (M = Ba, Sr, Ca) crystals and in other hosts with suitable environments. Copyright © 2021 Elsevier B.V. All rights reserved.