XEOL and Persistent Luminescence in Eu- and Ti-Doped Lu₂O₂S Materials

Rare-earth oxysulfides (RE2O2S, RE3+ = Y, La, Gd, Lu) are promising matrices for luminescent materials due to their high thermal and chemical stability, cost-effectiveness, and efficient sensitization of trivalent lanthanide ions, leading to high luminescent efficiency. These compounds crystallize in a trigonal structure, belonging to the space group P3̅m1. Lutetium oxysulfide (Lu2O2S) has been extensively studied as a host material for three-dimensional plasma display panels, field emission displays, and light-emitting diodes. Lu2O2S:Eu3+ exhibits red persistent luminescence, while Lu2O2S:Ti features a broad orange emission band associated with titanium. The incorporation of Mg2+ enhances afterglow duration by creating charge compensation defects and facilitating energy storage in trap levels. This work investigates the crystalline structure, optical absorption, and persistent luminescence properties of Lu2O2S and its doped variants: Lu2O2S:Eu3+, Lu2O2S:Ti, and Lu2O2S:Mg2+. Additionally, codoping effects were explored in Lu2O2S:Eu3+,Ti, Lu2O2S:Eu3+,Mg2+, Lu2O2S:Ti,Mg2+, and Lu2O2S:Eu3+,Ti,Mg2+. The materials were synthesized for the first time via a rapid and energy-efficient microwave-assisted solid-state method. Phase purity and crystal structure were analyzed by X-ray diffraction (XRD) with Rietveld refinement. The incorporation of Eu3+, Ti and Mg2+ was assessed along with resulting structural modifications. Lu2O2S band gap energy was obtained with Kubelka–Munk function on the diffuse reflectance spectroscopy (DRS) data. X-ray Absorption Near Edge Structure (XANES) and X-ray Excited Optical Luminescence (XEOL) measurements confirmed that absorption by the matrix at the Lu L3-edge effectively induces luminescence, playing a positive role in the emission mechanism. EPR spectra of Lu2O2S:Ti and Lu2O2S:Eu3+,Ti materials suggested that, even though Ti3+ might be present, photoredox processes are absent in the persistent luminescence mechanism and that Ti remains in the Ti4+ state. The observed visible-light emissions upon UV and X-ray excitation, along with the high energy storage capacity, highlight the potential of these materials for applications in dosimetry, bioimaging, and optoelectronic devices.

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