Structural Refinement, Core–Shell Electron Structure, Enhanced Phosphorescence, and Splitting of ⁵D₀ → ⁷F₂ Transitions in Li-doped Rare Earth Oxide Systems

Significant crystal field splitting and an enhancement in phosphorescence have been observed in the electric dipole transitions from ⁵D₀ to ⁷F₂ in Li-doped Eu³⁺:Gd₂O₃ compounds, which were synthesized using the high-temperature solid-state reaction technique. X-ray diffraction pattern analysis confirmed the formation of single-phase crystals, predominantly exhibiting the monoclinic phase characteristic of Gd₂O₃. Scanning electron microscopy and subsequent analysis demonstrated a dense distribution of microparticles, with average sizes ranging from 1.5 μm to 2 μm. The application of x-ray photoelectron spectroscopy facilitated the accurate identification of the constituent elements. The narrow-scan x-ray photoelectron spectra distinctly revealed the presence of Eu 3d, Li 1s, Gd 4d, and O 1s signals, and their deconvolution provided valuable insights into the stability of the valence states within the compound systems. Furthermore, the deep-level photoluminescence spectra indicated a significant increase in multiple emissions within the orange–red spectral range. These emissions, attributed to the transitions ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₁, and ⁵D₀ → ⁷F₀ states, are associated with the Eu activators, and the observed enhancement in intensity is linked to the crystallographic asymmetries introduced by Li ions.