Excitation-Selective Coexistence of Upconversion and Persistent Luminescence in Pr³⁺-doped Eulytite-Type Phosphates Towards Advanced Anti-Counterfeiting

Abstract

Pr3+-doped Ba3Lu(PO4)3 and Sr3Lu(PO4)3 phosphors exhibit multiple excitation-dependent luminescence modes and are studied as multifunctional materials for advanced anti-counterfeiting. When excited with blue light (444 nm), they efficiently produce blue-to-UVC upconversion via an excited-state absorption mechanism, while direct ultraviolet and visible excitation trigger characteristic Pr3+ photoluminescence. X-ray irradiation induces intense blue–red optical luminescence, followed by long-lasting red persistent luminescence detectable for several hours at room temperature. Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) measurements were employed to elucidate the nature of charge trapping processes that produce persistent emission. The TSL glow curves of all samples are dominated by a semi-broadband peak around 70 °C, indicative of a quasi-continuous distribution of trapping states. Tmax–Tstop experiments combined with initial rise analysis reveal trap depths between 0.9–1.15 eV, supporting effective room-temperature persistent luminescence (PersL). EPR spectroscopy identifies radiation-induced phosphorus-related radical centers and shallow traps, which most likely correspond to F+-type centers, with thermal stability matching TSL results. The coexistence of upconversion (UC), photoluminescence (PL), X-ray-induced optical luminescence (XRL), PersL, and TSL within a single Pr3+-doped host enables excitation-dependent color shifts and time-separated optical signals. These findings highlight the potential of Pr3+-doped eulytite-type phosphates as versatile, multi-level luminescent platforms for advanced anti-counterfeiting and encryption solutions.