Self-activated persistent phosphors, which exhibit intrinsic luminescence without requiring external dopants, hold significant potential for applications in optical storage, bioimaging, and safety signage due to their simplified synthesis and stable emission properties. However, the underlying mechanisms of their long afterglow behavior, particularly the role of intrinsic defects, remain poorly understood. We aim to explore the self-activated long afterglow phenomenon in Sr₃Y₂Ge₃O₁₂ (SYGO), a garnet-structured material, and elucidate the correlation between its luminescence characteristics and intrinsic defects. By analyzing defect types and their trapping mechanisms, we seek to establish a comprehensive model for defect-driven long afterglow, thus providing insights for designing novel self-activated phosphors.

SYGO samples are synthesized via solid-state reaction using SrCO₃, Y₂O₃, and GeO₂ as raw materials, calcined at 1200 ℃, 1250 ℃, and 1300 ℃ for 3 h. Phase purity and crystal structure are analyzed by X-ray diffraction (XRD, Rigaku D/Max-2500) with Cu-Kα radiation (λ=0.154 nm), and lattice parameters are refined using Rietveld refinement. High-resolution transmission electron microscopy (HRTEM, Hitachi S-4800) and selected-area electron diffraction (SAED) confirm the single-crystalline nature and garnet lattice. Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) verifies homogeneous distributions of Sr, Y, Ge, and O. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) reveals Sr2? (3d?_/? at 132.98 eV), Y3? (3d?_/? at 157.06 eV), and Ge?? (3d at 31.60 eV) as dominant oxidation states [Figs. 6(a)?(e)]. Photoluminescence (PL) spectra (Hitachi F-7000) and afterglow decay curves are measured under 254 nm excitation, with emission monitored at 570 nm (yellow) and 625 nm (red). Thermoluminescence (TL) glow curves (FJ-427A1 dosimeter) are recorded from 300 K to 600 K to analyze trap distribution. First-principle calculations based on density functional theory (DFT) are performed using the Vienna ab-initio simulation package (VASP) to compute defect formation energies and electronic structures.

SYGO exhibits temperature-dependent afterglow colors: yellow (1200 ℃), orange (1250 ℃), and red (1300 ℃), attributed to the interplay of intrinsic defects [Fig. 9(d)]. PL spectra show a broad emission band (555?640 nm) with peak shifts from 577 nm (yellow) to 625 nm (red) as calcination temperature increases [Figs. 10(a)?(d)]. XRD patterns confirm the cubic garnet structure (Ia-3d space group), with lattice constants expanding from 12.45 ? (1200 ℃) to 12.52 ? (1300 ℃) due to enhanced crystallinity and defect relaxation (Table 1). HRTEM images reveal lattice spacings of 0.1948 nm and 0.1528 nm, corresponding to (431) and (400) planes [Fig. 4(b)]. TL analysis identifies four distinct traps (T??T?) with depths of 0.757 eV, 0.838 eV, 0.946 eV, and 1.144 eV, assigned to VO··,VGe'''',VSr'' and SrY', respectively [Figs. 11(d)?(f), Table 4]. DFT calculations demonstrate that VO·· has the lowest formation energy (-2.1 eV), which acts as shallow electron traps, while VGe'''' (1.8 eV) and SrY' (1.5 eV) serve as deep hole traps [Fig. 11(b)]. Charge density difference plots [Fig. 13(a2)?(d2)] reveal localized electron accumulation around VO·· and hole localization at SrY', facilitating carrier recombination. Electron paramagnetic resonance (EPR) spectra [Fig. 14(a)] exhibit a g-factor of 2.003, characteristic of oxygen vacancy centers (VO··), whose intensity increases post-ultraviolet irradiation, confirming their role in charge storage.

We establish SYGO as a novel self-activated persistent phosphor with tunable afterglow colors governed by intrinsic defects. Through a synergy of experimental characterization and theoretical modeling, we demonstrate that oxygen vacancies (VO··), strontium vacancies (VSr''''), germanium vacancies (VGe''''), and antisite defects (SrY') collectively regulate carrier trapping and recombination. The proposed defect-mediated luminescence mechanism provides a blueprint for designing self-activated materials with tailored optical properties. Future studies will focus on optimizing defect concentrations via doping or annealing to achieve ultra-long afterglow durations (>24 h) for practical applications in emergency signage and in vivo imaging.