Radiation dose dependence of the Bi3+ luminescence in YPO4

Abstract

The UV persistent phosphors YPO₄:Bi were synthesized using both solid-state reaction and hydrothermal methods. XRD and SEM analyses confirmed the formation of single-phase powders. The solid-state synthesis resulted in ‘large’ particle sizes (1–4 μm), while the hydrothermal synthesis produced ‘small’ particle sizes (30–45 nm). The emission centers providing UV-C and UV-A luminescence under X-ray excitation are single Bi³⁺ ion and Bi³⁺– Bi³⁺ pairs. The highest emission yield was observed for samples with a Bi³⁺ concentration of 0.6 mol.%. An analysis of thermoluminescence glow curves using the general order kinetics model, revealed hole traps with an activation energy of 0.93–1.05 eV and electron traps with an activation energy of 0.75–0.85 eV. The dominant mechanisms for UV-C and UV-A emission involve hole recombination at Bi²⁺ ions and electron recombination at Bi3+– Bi⁴⁺ pairs, respectively. The optimal X-ray irradiation dose is 10–12 Gy. The UV-C and UV-A afterglow curves at room temperature follow the sum of exponential and first-order hyperbola functions, corresponding to electron recombination through the conduction band and hole recombination via tunneling, respectively. The afterglow durations for the solid-state prepared powder are 40 min for UV-A and 10 min for UV-C. The powder synthesized by hydrothermal method shows a significant reduction in intensity of afterglow due to the surface energy losses. Notably, despite the decreased intensity, there is a significant increase in the duration of the tunneling component responsible for UV-C emission, attributed to the reduced average distance between the emission center and the hole trap. Methods for improving the afterglow characteristics of YPO₄:Bi nanoparticles are proposed.