Correlation among photoluminescence and the electronic and atomic structures of Sr₂SiO₄:xEu³⁺ phosphors: X-ray absorption and emission studies

A series of Eu³⁺-activated strontium silicate phosphors, Sr₂SiO₄:xEu³⁺ (SSO:xEu³⁺, x = 1.0, 2.0 and 5.0%), were synthesized by a sol-gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic α'-SSO and monoclinic β-SSO phases of the SSO:xEu³⁺ phosphors. When SSO:xEu³⁺ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f-4f electronic transitions that involve the multiple excited ⁵D₀ → ⁷F_J levels (J = 1, 2, 3 and 4) of Eu³⁺ activators in the host matrix. This mechanism of PL in the SSO:xEu³⁺ phosphors is strongly related to the local electronic/atomic structures of the Eu³⁺-O^2- associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L₃-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and K_α X-ray emission spectroscopy. In the synthesis of SSO:xEu³⁺ phosphors, interstitial Eu₂O₃-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p-Eu 4f/5d states (mostly the O 2p-Eu 5d states) to the ⁵D₀ levels, facilitating the recombination of electrons that have transitioned from the ⁵D₀ level to the ⁷F_J level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu³⁺ phosphors. This PL-related behavior indicates that SSO:xEu³⁺ phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.