Irradiation-induced valence conversion of samarium ions in Na_2SO_4

摘要

Na_2SO_4:Sm phosphors were synthesized by heating pure sodium sulphate with a small amount of SmF_3 at 950 ℃. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements show that the particle size of as-synthesized phosphors is 58 and 60 nm, respectively. The samples were irradiated by ~(60)Co y-rays with irradiation doses of 10-50 kGy at room temperature and by electrons with a total fluence of 6.7 × 10~(16) e/cm~2 at a flux of 7.8 × 10~(11) e/cm~2·sec. After exposure to γ-ray and high energy electron irradiations, as-synthesized phosphors show the conversion of Sm~(3+)→ Sm~(2+). The pho-toluminescence (PL) spectra of as-synthesized, y-ray, and electron-irradiated phosphors obtained under 375 nm excitation consist of narrow emission bands with peaks at 563, 598, 644, and 704 nm, assigned to the ~4G(5/2)→~6Hy (J = 5/2,7/2,9/2, and 11/2) transitions within 4f~5 electronic configuration of Sm~(3+). The PL spectra of irradiatedrnphosphors under 570 nm excitation at room temperature consist of a broad band with a peak at 695 nm and an emission line at 685.9 nm on the broad band. The broad band is assigned to the 4f~5 5d → 4/~6 (~7Fy) transition and the emission line to the ~5Do→~7Fo transition within Sm~(2+). The PL and photoexcitation (PE) spectra at 10 K were well assigned to energy levels of Sm~(2+). Annealing at 473 K (200 ℃) completely erased the Sm~(2+) emission band within the detection limit at room temperature.