Topological Understanding of Luminescence in Eu²⁺-Doped Borate Phosphors

Eu²⁺-doped borates have been widely studied as LED phosphors, with a wide emission region from the ultraviolet to the infrared region, which is quite unusual for oxides. In this study, we focus on two representative Eu²⁺-doped borates, Ba₂Mg(BO₃)₂:Eu²⁺ and Ba₂Mg(B₃O₆)₂:Eu²⁺, to explore the underlying luminescent mechanisms: The former exhibits red emission, while the latter emits in the blue region. For this goal, the investigation is approached from two perspectives: (1) The crystal structures of the two borates are analyzed using topological crystallography, revealing a close structural relationship between perovskite and planar borates, which emphasizes the local environment surrounding the Eu²⁺ doping sites; (2) Ab-initio calculations were then performed to analyze in the atomic scale the origin of the different luminescence of the Eu²⁺ ion in the two borate compounds. The results indicated that the cation-cation interaction, namely, the 2p state of the B³⁺ ion and the 5d state of the Eu^2+,* ion, plays a dominant role in the red-shift of Eu²⁺-doped (BO₃)^3- compounds. These findings provide a deeper understanding of the photoluminescence of the 4f-5d transition, compared with the widely used Dorenbos model, and are expected to aid the development of phosphors.