Critical understanding on excitation-dependent multicolor emission from Eu³⁺ and Tb³⁺ co-doped SrF₂

Generating multicolor photoluminescence including white light from the same phosphor is advantageous while often a challenging task. Herein, a detailed study on excitation-dependent tunable photoluminescence characteristics in Eu3+ and Tb3+ co-doped SrF2 phosphor has been carried out. This phosphor yields lights of various colors including red, orange, and near white due to the change of the relative intensity of the blue, green, orange, and red color transitions of Eu3+ and Tb3+ ions. As revealed from DFT-based theoretical calculation, the existence of various mid-gap states created by the p orbitals of F- and f and d orbitals of Eu3+ and Tb3+ ions inside the wide band gap SrF2 host is responsible for the existence of multiple excitation peaks. The intensity of these excitation peaks is fully governed by the relative occupancy of these mid-gap states. This study helps future design of single phase multicolor emitting phosphor based lighting devices.