X-ray storage luminescence of BaFCl:Eu ²⁺ single crystals

Temperature behaviors of X-ray luminescence (XL), photoluminescence (PL), photostimulated luminescence, and thermoluminescence (TL) were studied in BaFCl:Eu ²⁺ single crystals from room temperature to liquid nitrogen temperature. Six emissions at 275, 315, 365, 385, 435, and 500 nm were observed in the XL spectra and are attributed to Cl excitons, V _k(Cl ₂ ^-), the 4f ⁶5d ¹ ( ²e _g) → 4f ⁷ ( ⁸S _7/2) transition of Eu ²⁺, and oxygen vacancies, respectively. Three emission peaks at 315, 365, and 390 nm were observed in the PL and TL measurements. These three emissions are from the transitions of 4f ⁷ ( ⁶I _7/2) → 4f ⁷ ( ⁸S _7/2), 4f ⁷ ( ⁶P _7/2) → 4f ⁷ ( ⁸S _7/2), and 4f ⁶5d ¹ ( ² _eg) → 4f ⁷( ⁸S _7/2) of Eu ²⁺, respectively. In our measurements, we observed that the emission of Eu ²⁺ increases in intensity upon β-irradiation and did not see any signals related to Eu ³⁺ ions, which indicates that Eu ²⁺ ions might not be oxidized to Eu ³⁺ upon X-ray or β-irradiation. Instead, the color centers, Cl excitons, and oxygen defects are created and are stable at room temperature, and they might play a key role in the storage luminescence.