Role of Paramagnetic Aluminum Hole Centers in UV–C Persistent Luminescence of Ca₂Al₂SiO₇:Pr³⁺

Materials with self-sustained emission in the ultraviolet (UV) spectral range present significant potential for practical applications. In this study, photochromic and persistent luminescence properties of Ca2Al2SiO7:Pr3+ are characterized by diffuse reflectance, photoluminescence, and thermally stimulated luminescence (TSL) spectroscopy methods. The material exhibits efficient persistent luminescence in the 250–350 nm range, with power density reaching 10.6 mW/m2 detected 10 s after 250 nm excitation and lasting for 3.7 h over the radiance threshold of 5 × 10–4 mW/m2/sr. In addition, photochromism is observed after either UV or X-ray irradiation. Multiple excitation cycles lead to noticeable coloration and reduced luminescence intensity, which can be restored by annealing. Electron paramagnetic resonance (EPR) spectroscopy indicates a correlation between persistent luminescence, photochromic properties, and paramagnetic centers in the material. The paramagnetic centers are identified as self-trapped holes at the Al(2) sites of the lattice, based on the g-factor and hyperfine interaction values determined from EPR and electron–nuclear double resonance (ENDOR) spectra simulations. These results provide a fundamental understanding of the structure–property relationship in Ca2Al2SiO7 and highlight practical considerations for developing UV–C persistent phosphor materials.