Fig. 1. XRD patterns and crystal structure of SYGO. (a) XRD patterns of SYGO at different calcination temperatures (1200‒1300 ℃); (b) enlarged views of SYGO in ranges of 26°‒35° and 47°‒53°; (c) XRD pattern of SYGO at 1300 ℃ (with an inset for the crystal lattice); (d) crystal structure of SYGO

Fig. 2. XRD pattern of SYGO at 1350 ℃

Fig. 3. SEM images of SYGO at different calcination temperatures. (a) 1200 ℃; (b) 1250 ℃; (c) 1300 ℃; (d) 1350 ℃

Fig. 4. Transmission electron microscopy (TEM) image and high-resolution transmission electron microscopy (HRTEM) image of SYGO, and surface scan images of different elements at calcination temperature of 1300 ℃. (a) TEM image, with an inset of the selected area electron diffraction (SAED) pattern; (b) HRTEM image; (c)‒(f) surface scan images of Sr, Y, Ge, and O elements

Fig. 5. EDS spectrum of SYGO at calcination temperature of 1300 ℃

Fig. 6. XPS full spectrum of SYGO and fitted spectra of different elements at calcination temperature of 1300 ℃. (a) XPS full spectrum; (b)‒(e) fitted spectra of O, Sr, Y, and Ge elements

Fig. 7. Diffuse reflectance spectrum and electronic structure of SYGO. (a) UV-visible diffuse reflectance absorption spectrum (with an inset showing the relationship between absorption coefficient and photon energy); (b) left: band structure; right: TDOS and PDOS

Fig. 8. PDOS of elements in SYGO. (a) O; (b) Sr; (c) Y; (d) Ge

Fig. 9. Luminescence decay curves of SYGO at different calcination temperatures (T) (inset images show luminescence decay photographs of SYGO). (a) λ_ex=254 nm, λ_em=570 nm; (b) λ_ex=254 nm, λ_em=625 nm

Fig. 10. Emission spectra fitting of SYGO at different calcination temperatures (T). (a) 1200 ℃; (b) 1250 ℃; (c) 1300 ℃; (d) emission spectra at different calcination temperatures

Fig. 11. Thermoluminescence characteristics and defect analysis of SYGO. (a) Thermoluminescence of SYGO at different calcination temperatures; (b) formation energies of point defects in SYGO; (c) distributions of defects in SYGO at different calcination temperatures; (d)‒(f) fitting curves of thermoluminescence peaks at different calcination temperatures

Fig. 12. Defect diagram of SYGO

Fig. 13. Different defect models and their differential charge distributions. (a1)‒(d1) Defect models for VO··,VSr″,VGe″″ and SrY'; (a2)‒(d2) differential charge distributions for VO··,VSr″,VGe″″ and SrY'

Fig. 14. EPR and differential EPR spectra of SGYO. (a) EPR spectra of SYGO before and after UV irradiation; (b) differential EPR spectra

Fig. 15. Schematic diagram of long afterglow mechanism of SYGO