Optical and Upconversion Properties of A₃Y(PO₄)₃:Pr³⁺ (A = Sr, Ba) Phosphors

The development of new ultraviolet (UV) phosphors is critical to unlocking the full potential of UV technologies to address global challenges related to healthcare, indoor air quality, and environmental sustainability. This work presents effective UV-C phosphors Sr3Y(PO4)3:Pr3+ and Ba3Y(PO4)3:Pr3+, synthesized by the solid-state method. The luminescent properties, including both down- and upconversion, were investigated under X-ray, UV, and visible excitation. Crystallites of A3Y(PO4)3 (A = Sr, Ba) activated by Pr3+ demonstrate effective ultraviolet 5d–4f emission and weak 4f–4f emission (mainly red 1D2 → 3H4 luminescence) upon ultraviolet excitation. Selective excitation of the 3PJ level of the Pr3+ ion in the crystallites leads to emissions in the blue, red-orange, and near-infrared regions, as well as broadband ultraviolet upconverted luminescence. The dependence of the integral intensity of Stokes and anti-Stokes emission on the concentration of the activator has been studied. The optimal concentrations of Pr3+ ions are determined to be 1 and 1.5 mol %, and the mechanisms of concentration quenching are discussed. To elucidate the upconversion mechanism, the intensity of upconversion emission was analyzed as a function of pumping. Ba3Y(PO4)3:Pr3+ shows a 6-fold increase in upconversion emission compared to YPO4:Pr3+ known from the literature, while Sr3Y(PO4)3:Pr3+ exhibits a 3.5-fold increase over the same material. The spectral-luminescent and luminescence-kinetic properties of pure and Pr3+-doped A3Y(PO4)3 (A = Sr, Ba) crystallites under excitation by X-rays up to 40 keV have been studied. The mechanism of energy transfer to Pr3+ ions in the crystallites upon X-ray excitation is discussed. Additionally, in the temperature range of 80–730 K, the optical thermometric characteristics of A3Y(PO4)3:Pr3+ crystallites were studied using the luminescence intensity ratio between two Pr3+ emission bands (3P1 → 3H5 and 3P0 → 3H6).